wlcore: add support macros to easily add conf debugfs entries

The current debugfs code contains too much code duplication
of bolierplate code. Add some macro magic to avoid this and
enable adding new debugfs entries by using just a few lines.

Signed-off-by: Eyal Shapira <eyal@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>