media: v4l2-ctrl: Add base layer priority id control.

This control indicates the priority id to be applied
to base layer.

[hverkuil: renumbered V4L2_CID_MPEG_VIDEO_BASELAYER_PRIORITY_ID]

Signed-off-by: Dikshita Agarwal <dikshita@codeaurora.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>