iio: adc: ad7768-1: add support for ADAQ776x-1 ADC Family

Add support for ADAQ7767/68/69-1 series, which includes PGIA and
Anti-aliasing filter (AAF) gains.

The PGA gain is configured in run-time through the scale attribute,
if supported by the device. The scale options are updated according
to the output data width.

The AAF gain is configured in the devicetree and it should correspond to
the AAF channel selected in hardware.

Signed-off-by: Jonathan Santos <[email protected]>