V4L/DVB: saa115: add support for chroma AGC and chroma gain

Add support for chroma agc and gain to the saa7115 driver.  This allows for
users to do manual adjustment for abnormal signal conditions.

This work was sponsored by EyeMagnet Limited.

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