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.
- nvmem-cells: A phandle to the calibration cells provided by eFuse device.
- nvmem-cell-names: Should be "big_scale_calib", "small_scale_calib".

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>;
			nvmem-cells = <&adc_big_scale>, <&adc_small_scale>;
			nvmem-cell-names = "big_scale_calib", "small_scale_calib";
		};
	};