media: uvcvideo: Add a D4M camera description

D4M is a mobile model from the D4XX family of Intel RealSense cameras.
This patch adds a descriptor for it, which enables reading per-frame
metadata from it.

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@intel.com>
[laurent.pinchart@ideasonboard.com Small clarifications to the documentation]
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>