iio: adc: adi-axi-adc: implement backend debugfs interface

Implement debugfs options to read/write registers and print the channel
status into a buffer (so we may know better the cause for errors) .

Signed-off-by: Nuno Sa <nuno.sa@analog.com>
Link: https://patch.msgid.link/20240802-dev-iio-backend-add-debugfs-v2-6-4cb62852f0d0@analog.com


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