[media] em28xx: add support for Terratec Grabby REC button

Terratec Grabby (hw rev 2) REC button uses the standard snapshot button
configuration.

Signed-off-by: Matthieu Rogez <matthieu.rogez@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>