[media] m920x: introduce m920x_rc_core_query()

Add an m920x_rc_core_query() function for drivers which want to use the
linux RC core infrastructure.

Signed-off-by: Antonio Ospite <ospite@studenti.unina.it>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>