[media] v4l: Add metadata buffer type and format

The metadata buffer type is used to transfer metadata between userspace
and kernelspace through a V4L2 buffers queue. It comes with a new
metadata capture capability and format description.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Tested-by: Guennadi Liakhovetski <guennadi.liakhovetski@intel.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
[hans.verkuil@cisco.com: removed left-over 'experimental' note]
[hans.verkuil@cisco.com: add newline after _v4l2-meta-format label]

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>