media: v4l: Add 4bpp packed depth confidence format CNF4

Adding new fourcc CNF4 for 4 bit-per-pixel packed depth confidence
information provided by Intel RealSense cameras. Every two consecutive
pixels are packed into a single byte.

Signed-off-by: Sergey Dorodnicov <sergey.dorodnicov@intel.com>
Signed-off-by: Evgeni Raikhel <evgeni.raikhel@intel.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>