Documentation: add the binding file for Freescale imx7d ADC driver

The patch adds the binding file for Freescale imx7d ADC driver.
Signed-off-by: Haibo Chen <haibo.chen@freescale.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

Documentation/devicetree/bindings/iio/adc/imx7d-adc.txt

+Freescale imx7d ADC bindings
+
+The devicetree bindings are for the ADC driver written for
+imx7d SoC.
+
+Required properties:
+- compatible: Should be "fsl,imx7d-adc"
+- reg: Offset and length of the register set for the ADC device
+- interrupts: The interrupt number for the ADC device
+- clocks: The root clock of the ADC controller
+- clock-names: Must contain "adc", matching entry in the clocks property
+- vref-supply: The regulator supply ADC reference voltage
+
+Example:
+adc1: adc@30610000 {
+	compatible = "fsl,imx7d-adc";
+	reg = <0x30610000 0x10000>;
+	interrupts = <GIC_SPI 98 IRQ_TYPE_LEVEL_HIGH>;
+	clocks = <&clks IMX7D_ADC_ROOT_CLK>;
+	clock-names = "adc";
+	vref-supply = <&reg_vcc_3v3_mcu>;
+};