Documentation: add the binding file for Freescale imx7d ADC driver

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>;

};