GPIO: MIPS: lantiq: convert gpio-mm-lantiq to OF and of_mm_gpio

Implements OF support and convert to of_mm_gpio.

By attaching hardware latches to the External Bus Unit (EBU) on Lantiq SoC, it
is possible to create output only gpios. This driver configures a special
memory address, which when written to, outputs 16 bit to the latches.

Signed-off-by: John Crispin <blogic@openwrt.org>
Cc: linux-kernel@vger.kernel.org
Cc: linux-mips@linux-mips.org
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Patchwork: https://patchwork.linux-mips.org/patch/3840/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

Documentation/devicetree/bindings/gpio/gpio-mm-lantiq.txt

@@ -0,0 +1,38 @@
+Lantiq SoC External Bus memory mapped GPIO controller
+
+By attaching hardware latches to the EBU it is possible to create output
+only gpios. This driver configures a special memory address, which when
+written to outputs 16 bit to the latches.
+
+The node describing the memory mapped GPIOs needs to be a child of the node
+describing the "lantiq,localbus".
+
+Required properties:
+- compatible : Should be "lantiq,gpio-mm-lantiq"
+- reg : Address and length of the register set for the device
+- #gpio-cells : Should be two.  The first cell is the pin number and
+  the second cell is used to specify optional parameters (currently
+  unused).
+- gpio-controller : Marks the device node as a gpio controller.
+
+Optional properties:
+- lantiq,shadow : The default value that we shall assume as already set on the
+  shift register cascade.
+
+Example:
+
+localbus@0 {
+	#address-cells = <2>;
+	#size-cells = <1>;
+	ranges = <0 0 0x0 0x3ffffff /* addrsel0 */
+		1 0 0x4000000 0x4000010>; /* addsel1 */
+	compatible = "lantiq,localbus", "simple-bus";
+
+	gpio_mm0: gpio@4000000 {
+		compatible = "lantiq,gpio-mm";
+		reg = <1 0x0 0x10>;
+		gpio-controller;
+		#gpio-cells = <2>;
+		lantiq,shadow = <0x77f>
+	};
+}