[media] v4l2: add RF gain control

Add new RF tuner gain control named RF Gain. That is aimed for first
amplifier chip right after antenna connector.
There is existing LNA Gain control, which is quite same, but it is
aimed for cases amplifier is integrated to tuner chip. Some designs
have both, as almost all recent tuner silicons has integrated LNA/RF
amplifier in any case.

Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>