iio: backend: add support for filter config

iio: backend: add support for filter config

Add backend support for digital filter type selection.

This setting can be adjusted within the IP cores interfacing devices.

The IP core can be configured based on the state of the actual
digital filter configuration of the part.

Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Signed-off-by: Antoniu Miclaus <antoniu.miclaus@analog.com>
Link: https://patch.msgid.link/20250516082630.8236-2-antoniu.miclaus@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>