i2c: uniphier: add UniPhier FIFO-less I2C driver

Add support for on-chip I2C controller used on old UniPhier SoCs
such as PH1-LD4, PH1-sLD8, etc.  This adapter is so simple that
it has no FIFO in it.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>

Documentation/devicetree/bindings/i2c/i2c-uniphier.txt

@@ -0,0 +1,25 @@
+UniPhier I2C controller (FIFO-less)
+
+Required properties:
+- compatible: should be "socionext,uniphier-i2c".
+- #address-cells: should be 1.
+- #size-cells: should be 0.
+- reg: offset and length of the register set for the device.
+- interrupts: a single interrupt specifier.
+- clocks: phandle to the input clock.
+
+Optional properties:
+- clock-frequency: desired I2C bus frequency in Hz.  The maximum supported
+  value is 400000.  Defaults to 100000 if not specified.
+
+Examples:
+
+	i2c0: i2c@58400000 {
+		compatible = "socionext,uniphier-i2c";
+		reg = <0x58400000 0x40>;
+		#address-cells = <1>;
+		#size-cells = <0>;
+		interrupts = <0 41 1>;
+		clocks = <&i2c_clk>;
+		clock-frequency = <100000>;
+	};