Freescale vf610 Analog to Digital Converter bindings

The devicetree bindings are for the new ADC driver written for
vf610/i.MX6slx and upward SoCs from Freescale.

Required properties:
- compatible: Should contain "fsl,vf610-adc"
- reg: Offset and length of the register set for the device
- interrupts: Should contain the interrupt for the device
- clocks: The clock is needed by the ADC controller, ADC clock source is ipg clock.
- clock-names: Must contain "adc", matching entry in the clocks property.
- vref-supply: The regulator supply ADC reference voltage.

Recommended properties:
- fsl,adck-max-frequency: Maximum frequencies according to datasheets operating
  requirements. Three values are required, depending on conversion mode:
  - Frequency in normal mode (ADLPC=0, ADHSC=0)
  - Frequency in high-speed mode (ADLPC=0, ADHSC=1)
  - Frequency in low-power mode (ADLPC=1, ADHSC=0)
- min-sample-time: Minimum sampling time in nanoseconds. This value has
  to be chosen according to the conversion mode and the connected analog
  source resistance (R_as) and capacitance (C_as). Refer the datasheet's
  operating requirements. A safe default across a wide range of R_as and
  C_as as well as conversion modes is 1000ns.

Example:
adc0: adc@4003b000 {
	compatible = "fsl,vf610-adc";
	reg = <0x4003b000 0x1000>;
	interrupts = <0 53 0x04>;
	clocks = <&clks VF610_CLK_ADC0>;
	clock-names = "adc";
	fsl,adck-max-frequency = <30000000>, <40000000>,
				<20000000>;
	vref-supply = <&reg_vcc_3v3_mcu>;
};