FROMGIT: media: v4l2-ctrl: add controls for long term reference.

Long Term Reference (LTR) frames are the frames that are encoded
sometime in the past and stored in the DPB buffer list to be used
as reference to encode future frames.
This change adds controls to enable this feature.

Signed-off-by: Dikshita Agarwal <dikshita@codeaurora.org>
Reviewed-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Stanimir Varbanov <stanimir.varbanov@linaro.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
(cherry picked from commit f15c54cf
 git://linuxtv.org/media_tree.git master)
Bug: 175378287
Link: https://lkml.org/lkml/2021/3/24/368



Change-Id: I4890af1665d33ba16a1bf9ae96b8d431ecedd06e
Signed-off-by: Dikshita Agarwal <dikshita@codeaurora.org>