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

This patch adds the binding documentation for Spreadtrum SC27XX series
PMICs ADC controller device.
Signed-off-by: NBaolin Wang <baolin.wang@linaro.org>
Reviewed-by: NRob Herring <robh@kernel.org>
Signed-off-by: NJonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/devicetree/bindings/iio/adc/sprd,sc27xx-adc.txt

+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>;
+		};
+	};