iio: backend: add debugFs interface

This adds a basic debugfs interface for backends. Two new ops are being
added:

 * debugfs_reg_access: Analogous to the core IIO one but for backend
   devices.
 * debugfs_print_chan_status: One useful usecase for this one is for
   testing test tones in a digital interface and "ask" the backend to
   dump more details on why a test tone might have errors.

Signed-off-by: Nuno Sa <nuno.sa@analog.com>
Link: https://patch.msgid.link/20240802-dev-iio-backend-add-debugfs-v2-2-4cb62852f0d0@analog.com


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