media: v4l: Add definitions for MPEG-2 slice format and metadata

Stateless video decoding engines require both the MPEG-2 slices and
associated metadata from the video stream in order to decode frames.

This introduces definitions for a new pixel format, describing buffers
with MPEG-2 slice data, as well as control structure sfor passing the
frame metadata to drivers.

This is based on work from both Florent Revest and Hugues Fruchet.

Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>