staging: iio: ad7780: add device tree binding

* Analog Devices AD7170/AD7171/AD7780/AD7781

Data sheets:

- AD7170:

	* https://www.analog.com/media/en/technical-documentation/data-sheets/AD7170.pdf

- AD7171:

	* https://www.analog.com/media/en/technical-documentation/data-sheets/AD7171.pdf

- AD7780:

	* https://www.analog.com/media/en/technical-documentation/data-sheets/ad7780.pdf

- AD7781:

	* https://www.analog.com/media/en/technical-documentation/data-sheets/AD7781.pdf

Required properties:

- compatible: should be one of

	* "adi,ad7170"

	* "adi,ad7171"

	* "adi,ad7780"

	* "adi,ad7781"

- reg: spi chip select number for the device

- vref-supply: the regulator supply for the ADC reference voltage

Optional properties:

- powerdown-gpios:  must be the device tree identifier of the PDRST pin. If

		    specified, it will be asserted during driver probe. As the

		    line is active high, it should be marked GPIO_ACTIVE_HIGH.

- adi,gain-gpios:   must be the device tree identifier of the GAIN pin. Only for

		    the ad778x chips. If specified, it will be asserted during

		    driver probe. As the line is active low, it should be marked

		    GPIO_ACTIVE_LOW.

- adi,filter-gpios: must be the device tree identifier of the FILTER pin. Only

		    for the ad778x chips. 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,ad7780";

	reg =	      <0>;

	vref-supply = <&vdd_supply>

	powerdown-gpios  = <&gpio 12 GPIO_ACTIVE_HIGH>;

	adi,gain-gpios   = <&gpio  5 GPIO_ACTIVE_LOW>;

	adi,filter-gpios = <&gpio 15 GPIO_ACTIVE_LOW>;

};