iio: adc: ad9467: add digital interface test to debugfs

One useful thing to do (in case of problems) in this high speed devices
with digital interfaces is to try different test patterns to see if the
interface is working properly (and properly calibrated). Hence add this
to debugfs.

On top of this, for some test patterns, the backend may have a matching
validator block which can be helpful in identifying possible issues. For
the other patterns some test equipment must be used so one can look into
the signal and see how it looks like.

Hence, we also add the backend debugfs interface with
iio_backend_debugfs_add().

Signed-off-by: Nuno Sa <nuno.sa@analog.com>
Link: https://patch.msgid.link/20240802-dev-iio-backend-add-debugfs-v2-8-4cb62852f0d0@analog.com


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