iio: dac: adi-axi-dac: support debugfs direct_reg_access

Make it possible to read/write registers on the backend using the
debugfs interface for it.

Signed-off-by: Nuno Sa <nuno.sa@analog.com>
Link: https://patch.msgid.link/20240806-dev-backend-dac-direct-reg-access-v1-1-b84a6e8ee8a0@analog.com


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