media: v4l2-ctrl: Add control for h.264 chroma qp offset

Allow to add fixed quantization parameter offset between luma and
chroma quantization parameters. This control directly corresponds
to the chroma_qp_index_offset field of the h.264 picture parameter
set.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>