media: venus: venc: Add support for Long Term Reference (LTR) controls

Add support for below LTR controls in encoder:
- V4L2_CID_MPEG_VIDEO_LTR_COUNT
- V4L2_CID_MPEG_VIDEO_FRAME_LTR_INDEX
- V4L2_CID_MPEG_VIDEO_USE_LTR_FRAMES

Signed-off-by: Dikshita Agarwal <dikshita@codeaurora.org>
Signed-off-by: Stanimir Varbanov <stanimir.varbanov@linaro.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>