[media] media: add messages when media device gets (un)registered

We can only free the media device after being sure that no
graph object is used.

In order to help tracking it, let's add debug messages
that will print when the media controller gets registered
or unregistered.

Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Reviewed-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>