i2c: cadence: Document device tree bindings

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>;

	};