iio: adc: update ad7779 to use IIO backend

Add a new functionality to ad7779 driver that streams data through data
output interface using IIO backend interface.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Signed-off-by: Ioana Risteiu <Ioana.Risteiu@analog.com>
Link: https://patch.msgid.link/20250825221355.6214-5-Ioana.Risteiu@analog.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>