Add RGB444_1X12 and RGB565_1X16 media bus formats

Add RGB444_1X12 and RGB565_1X16 format definitions and update the
documentation.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>