iio: adc: adi-axi-adc: support digital interface calibration

Implement the new IIO backend APIs for calibrating the data
digital interfaces.

While at it, removed the tabs in 'struct adi_axi_adc_state' and used
spaces for the members.

Signed-off-by: Nuno Sa <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20240426-ad9467-new-features-v2-6-6361fc3ba1cc@analog.com


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