Analog Devices AD7768-1 ADC device driver

Required properties for the AD7768-1:

- compatible: Must be "adi,ad7768-1"
- reg: SPI chip select number for the device
- spi-max-frequency: Max SPI frequency to use
	see: Documentation/devicetree/bindings/spi/spi-bus.txt
- clocks: phandle to the master clock (mclk)
	see: Documentation/devicetree/bindings/clock/clock-bindings.txt
- clock-names: Must be "mclk".
- interrupts: IRQ line for the ADC
	see: Documentation/devicetree/bindings/interrupt-controller/interrupts.txt
- vref-supply: vref supply can be used as reference for conversion
- adi,sync-in-gpios: must be the device tree identifier of the SYNC-IN pin. Enables
	synchronization of multiple devices that require simultaneous sampling.
	A pulse is always required if the configuration is changed in any way, for example
	if the filter decimation rate changes. As the line is active low, it should
	be marked GPIO_ACTIVE_LOW.

Optional properties:

 - reset-gpios : GPIO spec for the RESET pin. If specified, it will be asserted during
	driver probe. As the line is active low, it should be marked GPIO_ACTIVE_LOW.

Example:

	adc@0 {
		compatible = "adi,ad7768-1";
		reg = <0>;
		spi-max-frequency = <2000000>;
		spi-cpol;
		spi-cpha;
		vref-supply = <&adc_vref>;
		interrupts = <25 IRQ_TYPE_EDGE_RISING>;
		interrupt-parent = <&gpio>;
		adi,sync-in-gpios = <&gpio 22 GPIO_ACTIVE_LOW>;
		reset-gpios = <&gpio 27 GPIO_ACTIVE_LOW>;
		clocks = <&ad7768_mclk>;
		clock-names = "mclk";
	};