staging: pi433: add debugfs interface

staging: pi433: add debugfs interface

This adds debugfs interface that can be used for debugging possible
hardware/software issues.

It currently exposes the following debugfs entries for each SPI device
probed:

  /sys/kernel/debug/pi433/<DEVICE>/regs
  ...

The 'regs' file contains all rf69 uC registers values that are useful
for troubleshooting misconfigurations between 2 devices. It contains one
register per line so it should be easy to use normal filtering tools to
find the registers of interest if needed.

Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Paulo Miguel Almeida <paulo.miguel.almeida.rodenas@gmail.com>
Link: https://lore.kernel.org/r/YfzeEHJcd+qvYGZ8@mail.google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>