iio: adc: adi-axi-adc: move to backend framework

Move to the IIO backend framework. Devices supported by adi-axi-adc now
register themselves as backend devices.

Signed-off-by: Nuno Sa <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20240210-iio-backend-v11-7-f5242a5fb42a@analog.com


Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>