dt-bindings: document Rockchip saradc

Rockchip Successive Approximation Register (SAR) A/D Converter bindings

Required properties:

- compatible: Should be "rockchip,saradc"

- reg: physical base address of the controller and length of memory mapped

       region.

- interrupts: The interrupt number to the cpu. The interrupt specifier format

              depends on the interrupt controller.

- clocks: Must contain an entry for each entry in clock-names.

- clock-names: Shall be "saradc" for the converter-clock, and "apb_pclk" for

               the peripheral clock.

- vref-supply: The regulator supply ADC reference voltage.

- #io-channel-cells: Should be 1, see ../iio-bindings.txt

Example:

	saradc: saradc@2006c000 {

		compatible = "rockchip,saradc";

		reg = <0x2006c000 0x100>;

		interrupts = <GIC_SPI 26 IRQ_TYPE_LEVEL_HIGH>;

		clocks = <&cru SCLK_SARADC>, <&cru PCLK_SARADC>;

		clock-names = "saradc", "apb_pclk";

		#io-channel-cells = <1>;

		vref-supply = <&vcc18>;

	};