media: cedrus: Specify H264 startcode and decoding mode

The cedrus VPU is slice-based and expects V4L2_PIX_FMT_H264_SLICE
buffers to contain H264 slices with no start code.

Expose this to userspace with the newly added menu control.

These two controls are specified as mandatory for applications,
but we mark them as non-required on the driver side for
backwards compatibility.

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>