i2c: cadence: Document device tree bindings

Add device tree binding documentation for the Cadence I2C controller.
Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>

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

+Binding for the Cadence I2C controller
+
+Required properties:
+  - reg: Physical base address and size of the controller's register area.
+  - compatible: Compatibility string. Must be 'cdns,i2c-r1p10'.
+  - clocks: Input clock specifier. Refer to common clock bindings.
+  - interrupts: Interrupt specifier. Refer to interrupt bindings.
+  - #address-cells: Should be 1.
+  - #size-cells: Should be 0.
+
+Optional properties:
+  - clock-frequency: Desired operating frequency, in Hz, of the bus.
+  - clock-names: Input clock name, should be 'pclk'.
+
+Example:
+	i2c@e0004000 {
+		compatible = "cdns,i2c-r1p10";
+		clocks = <&clkc 38>;
+		interrupts = <GIC_SPI 25 IRQ_TYPE_LEVEL_HIGH>;
+		reg = <0xe0004000 0x1000>;
+		clock-frequency = <400000>;
+		#address-cells = <1>;
+		#size-cells = <0>;
+	};