dt-bindings: iio: adc: ad7768-1: add new supported parts

dt-bindings: iio: adc: ad7768-1: add new supported parts

Add compatibles for supported parts in the ad7768-1 family:
	ADAQ7767-1, ADAQ7768-1 and ADAQ7769-1

Add property and checks for AFF gain, supported by ADAQ7767-1
and ADAQ7769-1, and for PGA gain, supported by ADAQ7768-1
and ADAQ7769-1:
	adi,aaf-gain-bp
	pga-gpios

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
Signed-off-by: Jonathan Santos <Jonathan.Santos@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>