drivers: iio: adc: add support for ad777x family

drivers: iio: adc: add support for ad777x family

Add support for AD7770, AD7771, AD7779 ADCs. The device is capable of
sending out data both on DOUT lines interface,as on the SDO line.
The driver currently implements only the SDO data streaming mode. SPI
communication is used alternatively for accessing registers and streaming
data. Register accesses are protected by crc8.

Signed-off-by: Ramona Alexandra Nechita <ramona.nechita@analog.com>
Link: https://patch.msgid.link/20241014143204.30195-4-ramona.nechita@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>