media: v4l2-ctrls: Add average QP control

Add a control V4L2_CID_MPEG_VIDEO_AVERAGE_QP to report the average QP
value of the current encoded frame. The value applies to the last
dequeued capture buffer.

Signed-off-by: Ming Qian <ming.qian@nxp.com>
Reviewed-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Sebastian Fricke <sebastian.fricke@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>