[media] media: rc: document rc class sysfs API

Briefly document /sys/class/rc/ API for remote controller devices in
Documentation/ABI/teting.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Rob Landley <rob@landley.net>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>