dt-bindings: iio: Add Spreadtrum SC27XX PMICs ADC controller documentation

Spreadtrum SC27XX series PMICs ADC binding

Required properties:

- compatible: Should be one of the following.

	"sprd,sc2720-adc"

	"sprd,sc2721-adc"

	"sprd,sc2723-adc"

	"sprd,sc2730-adc"

	"sprd,sc2731-adc"

- reg: The address offset of ADC controller.

- interrupt-parent: The interrupt controller.

- interrupts: The interrupt number for the ADC device.

- #io-channel-cells: Number of cells in an IIO specifier.

- hwlocks: Reference to a phandle of a hwlock provider node.

Example:

	sc2731_pmic: pmic@0 {

		compatible = "sprd,sc2731";

		reg = <0>;

		spi-max-frequency = <26000000>;

		interrupts = <GIC_SPI 31 IRQ_TYPE_LEVEL_HIGH>;

		interrupt-controller;

		#interrupt-cells = <2>;

		#address-cells = <1>;

		#size-cells = <0>;

		pmic_adc: adc@480 {

			compatible = "sprd,sc2731-adc";

			reg = <0x480>;

			interrupt-parent = <&sc2731_pmic>;

			interrupts = <0 IRQ_TYPE_LEVEL_HIGH>;

			#io-channel-cells = <1>;

			hwlocks = <&hwlock 4>;

		};

	};