V4L/DVB: videodev2: introduce a common control for chroma gain

Introduce a new control for modifying the chroma gain.  This allows for user
intervention in abnormal signal conditions cases where the decoder's chroma
AGC cannot compensate and the value needs to be adjusted manually.

This work was sponsored by EyeMagnet Limited.

Signed-off-by: Devin Heitmueller <dheitmueller@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>