clk: add DT clock binding support

Based on work 1st by Ben Herrenschmidt and Jeremy Kerr, then by Grant
Likely, this patch adds support to clk_get to allow drivers to retrieve
clock data from the device tree.

Platforms scan for clocks in DT with of_clk_init and a match table, and
the register a provider through of_clk_add_provider. The provider's
clk_src_get function will be called when a device references the
provider's OF node for a clock reference.

v6 (Rob Herring):
    - Return error values instead of NULL to match clock framework
      expectations

v5 (Rob Herring):
    - Move from drivers/of into common clock subsystem
    - Squashed "dt/clock: add a simple provider get function" and
      "dt/clock: add function to get parent clock name"
    - Rebase to 3.4-rc1
    - Drop CONFIG_OF_CLOCK and just use CONFIG_OF
    - Add missing EXPORT_SYMBOL to various functions
    - s/clock-output-name/clock-output-names/
    - Define that fixed-clock binding is a single output

v4 (Rob Herring):
    - Rework for common clk subsystem
    - Add of_clk_get_parent_name function

v3: - Clarified documentation

v2: - fixed errant ';' causing compile error
    - Editorial fixes from Shawn Guo
    - merged in adding lookup to clkdev
    - changed property names to match established convention. After
      working with the binding a bit it really made more sense to follow the
      lead of 'reg', 'gpios' and 'interrupts' by making the input simply
      'clocks' & 'clock-names' instead of 'clock-input-*', and to only use
      clock-output* for the producer nodes. (Sorry Shawn, this will mean
      you need to change some code, but it should be trivial)
    - Add ability to inherit clocks from parent nodes by using an empty
      'clock-ranges' property.  Useful for busses.  I could use some feedback
      on the new property name, 'clock-ranges' doesn't feel right to me.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Reviewed-by: Shawn Guo <shawn.guo@freescale.com>
Cc: Sascha Hauer <kernel@pengutronix.de>
Signed-off-by: Mike Turquette <mturquette@linaro.org>

Documentation/devicetree/bindings/clock/clock-bindings.txt

@@ -0,0 +1,117 @@
+This binding is a work-in-progress, and are based on some experimental
+work by benh[1].
+
+Sources of clock signal can be represented by any node in the device
+tree.  Those nodes are designated as clock providers.  Clock consumer
+nodes use a phandle and clock specifier pair to connect clock provider
+outputs to clock inputs.  Similar to the gpio specifiers, a clock
+specifier is an array of one more more cells identifying the clock
+output on a device.  The length of a clock specifier is defined by the
+value of a #clock-cells property in the clock provider node.
+
+[1] http://patchwork.ozlabs.org/patch/31551/
+
+==Clock providers==
+
+Required properties:
+#clock-cells:	   Number of cells in a clock specifier; Typically 0 for nodes
+		   with a single clock output and 1 for nodes with multiple
+		   clock outputs.
+
+Optional properties:
+clock-output-names: Recommended to be a list of strings of clock output signal
+		    names indexed by the first cell in the clock specifier.
+		    However, the meaning of clock-output-names is domain
+		    specific to the clock provider, and is only provided to
+		    encourage using the same meaning for the majority of clock
+		    providers.  This format may not work for clock providers
+		    using a complex clock specifier format.  In those cases it
+		    is recommended to omit this property and create a binding
+		    specific names property.
+
+		    Clock consumer nodes must never directly reference
+		    the provider's clock-output-names property.
+
+For example:
+
+    oscillator {
+        #clock-cells = <1>;
+        clock-output-names = "ckil", "ckih";
+    };
+
+- this node defines a device with two clock outputs, the first named
+  "ckil" and the second named "ckih".  Consumer nodes always reference
+  clocks by index. The names should reflect the clock output signal
+  names for the device.
+
+==Clock consumers==
+
+Required properties:
+clocks:		List of phandle and clock specifier pairs, one pair
+		for each clock input to the device.  Note: if the
+		clock provider specifies '0' for #clock-cells, then
+		only the phandle portion of the pair will appear.
+
+Optional properties:
+clock-names:	List of clock input name strings sorted in the same
+		order as the clocks property.  Consumers drivers
+		will use clock-names to match clock input names
+		with clocks specifiers.
+clock-ranges:	Empty property indicating that child nodes can inherit named
+		clocks from this node. Useful for bus nodes to provide a
+		clock to their children.
+
+For example:
+
+    device {
+        clocks = <&osc 1>, <&ref 0>;
+        clock-names = "baud", "register";
+    };
+
+
+This represents a device with two clock inputs, named "baud" and "register".
+The baud clock is connected to output 1 of the &osc device, and the register
+clock is connected to output 0 of the &ref.
+
+==Example==
+
+    /* external oscillator */
+    osc: oscillator {
+        compatible = "fixed-clock";
+        #clock-cells = <1>;
+        clock-frequency  = <32678>;
+        clock-output-names = "osc";
+    };
+
+    /* phase-locked-loop device, generates a higher frequency clock
+     * from the external oscillator reference */
+    pll: pll@4c000 {
+        compatible = "vendor,some-pll-interface"
+        #clock-cells = <1>;
+        clocks = <&osc 0>;
+        clock-names = "ref";
+        reg = <0x4c000 0x1000>;
+        clock-output-names = "pll", "pll-switched";
+    };
+
+    /* UART, using the low frequency oscillator for the baud clock,
+     * and the high frequency switched PLL output for register
+     * clocking */
+    uart@a000 {
+        compatible = "fsl,imx-uart";
+        reg = <0xa000 0x1000>;
+        interrupts = <33>;
+        clocks = <&osc 0>, <&pll 1>;
+        clock-names = "baud", "register";
+    };
+
+This DT fragment defines three devices: an external oscillator to provide a
+low-frequency reference clock, a PLL device to generate a higher frequency
+clock signal, and a UART.
+
+* The oscillator is fixed-frequency, and provides one clock output, named "osc".
+* The PLL is both a clock provider and a clock consumer. It uses the clock
+  signal generated by the external oscillator, and provides two output signals
+  ("pll" and "pll-switched").
+* The UART has its baud clock connected the external oscillator and its
+  register clock connected to the PLL clock (the "pll-switched" signal)

Documentation/devicetree/bindings/clock/fixed-clock.txt

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+Binding for simple fixed-rate clock sources.
+
+This binding uses the common clock binding[1].
+
+[1] Documentation/devicetree/bindings/clock/clock-bindings.txt
+
+Required properties:
+- compatible : shall be "fixed-clock".
+- #clock-cells : from common clock binding; shall be set to 0.
+- clock-frequency : frequency of clock in Hz. Should be a single cell.
+
+Optional properties:
+- gpios : From common gpio binding; gpio connection to clock enable pin.
+- clock-output-names : From common clock binding.
+
+Example:
+	clock {
+		compatible = "fixed-clock";
+		#clock-cells = <0>;
+		clock-frequency = <1000000000>;
+	};