iio: backend: add support for data source get

Add backend support for getting the data source used.

The ad3552r HDL implements an internal ramp generator, so adding the
getter to allow data source get/set by debugfs.

Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Signed-off-by: Angelo Dureghello <adureghello@baylibre.com>
Link: https://patch.msgid.link/20250409-wip-bl-ad3552r-fixes-v5-3-fb429c3a6515@baylibre.com


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