V4L/DVB (7450): v4l2-api: Define a standard control for chroma AGC

Define a pre-defined control ID for chroma automatic gain control.

Signed-off-by: "Frej Drejhammar <frej.drejhammar@gmail.com&gt;">
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>