iio: adc: Add AD7768-1 ADC basic support

The ad7768-1 is a single channel, precision 24-bit analog to digital
converter (ADC).

This basic patch configures the device in fast mode, with 32 kSPS and
leaves the default sinc5 filter.

Two data conversion modes are made available. When data is retrieved by
using the read_raw attribute, one shot single conversion mode is set.
The continuous conversion mode is enabled when the triggered buffer
mechanism is used. To assure correct data retrieval, the driver waits
for the interrupt triggered by the low to high transition of the DRDY
pin.

Datasheets:
Link: https://www.analog.com/media/en/technical-documentation/data-sheets/ad7768-1.pdf



Signed-off-by: Stefan Popa <stefan.popa@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>