dt-bindings: document Rockchip saradc

This add the necessary binding documentation for the saradc found in all recent
processors from Rockchip.
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

Documentation/devicetree/bindings/iio/adc/rockchip-saradc.txt

+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>;
+	};