Analog Devices AD7606 Simultaneous Sampling ADC

Required properties for the AD7606:

- compatible: Must be one of
	* "adi,ad7605-4"
	* "adi,ad7606-8"
	* "adi,ad7606-6"
	* "adi,ad7606-4"
- reg: SPI chip select number for the device
- spi-max-frequency: Max SPI frequency to use
	see: Documentation/devicetree/bindings/spi/spi-bus.txt
- spi-cpha: See Documentation/devicetree/bindings/spi/spi-bus.txt
- avcc-supply: phandle to the Avcc power supply
- interrupts: IRQ line for the ADC
	see: Documentation/devicetree/bindings/interrupt-controller/interrupts.txt
- adi,conversion-start-gpios: must be the device tree identifier of the CONVST pin.
		  This logic input is used to initiate conversions on the analog
		  input channels. As the line is active high, it should be marked
		  GPIO_ACTIVE_HIGH.

Optional properties:

- reset-gpios: must be the device tree identifier of the RESET pin. If specified,
	       it will be asserted during driver probe. As the line is active high,
	       it should be marked GPIO_ACTIVE_HIGH.
- standby-gpios: must be the device tree identifier of the STBY pin. This pin is used
		to place the AD7606 into one of two power-down modes, Standby mode or
		Shutdown mode. As the line is active low, it should be marked
		GPIO_ACTIVE_LOW.
- adi,first-data-gpios: must be the device tree identifier of the FRSTDATA pin.
		    The FRSTDATA output indicates when the first channel, V1, is
		    being read back on either the parallel, byte or serial interface.
		    As the line is active high, it should be marked GPIO_ACTIVE_HIGH.
- adi,range-gpios: must be the device tree identifier of the RANGE pin. The polarity on
	      this pin determines the input range of the analog input channels. If
	      this pin is tied to a logic high, the analog input range is ±10V for
	      all channels. If this pin is tied to a logic low, the analog input range
	      is ±5V for all channels. As the line is active high, it should be marked
	      GPIO_ACTIVE_HIGH.
- adi,oversampling-ratio-gpios: must be the device tree identifier of the over-sampling
				mode pins. As the line is active high, it should be marked
				GPIO_ACTIVE_HIGH.

Example:

	adc@0 {
		compatible = "adi,ad7606-8";
		reg = <0>;
		spi-max-frequency = <1000000>;
		spi-cpol;

		avcc-supply = <&adc_vref>;

		interrupts = <25 IRQ_TYPE_EDGE_FALLING>;
		interrupt-parent = <&gpio>;

		adi,conversion-start-gpios = <&gpio 17 GPIO_ACTIVE_HIGH>;
		reset-gpios = <&gpio 27 GPIO_ACTIVE_HIGH>;
		adi,first-data-gpios = <&gpio 22 GPIO_ACTIVE_HIGH>;
		adi,oversampling-ratio-gpios = <&gpio 18 GPIO_ACTIVE_HIGH
						&gpio 23 GPIO_ACTIVE_HIGH
						&gpio 26 GPIO_ACTIVE_HIGH>;
		standby-gpios = <&gpio 24 GPIO_ACTIVE_LOW>;
	};