[media] V4L: add Y12I, Y8I and Z16 pixel format documentation

Add documentation for 3 formats, used by RealSense cameras like R200.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>