media: pixfmt: Add H264 Slice format

The H264_SLICE_RAW format is meant to hold the parsed slice data without
the start code. This will be needed by stateless decoders.

Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>