Add support for base layer priority ID control in encoder.
This is a preparation patch to support v6.

[hverkuil: changed 54 to 51 in v4l2_ctrl_handler_init]
Signed-off-by: Dikshita Agarwal <dikshita@codeaurora.org>
Acked-by: Stanimir Varbanov <stanimir.varbanov@linaro.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>