media: Documentation: mc: Add media_device_{init,cleanup}

Document that drivers must first initialise a media device before
registering it, and clean up once the media device is unregistered.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>