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
- #io-channel-cells: Must be 1 as per ../iio-bindings.txt

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>;
	#io-channel-cells = <1>;
};