[media] v4l: Add ARGB555X and XRGB555X pixel formats

The existing RGB555X pixel format is ill-defined in respect to its alpha
bit and its meaning is driver dependent. Create new standard ARGB555X
and XRGB555X variants with clearly defined meanings and make the
existing variant deprecated.

The new pixel formats 4CC values have been selected to match the DRM
4CCs for the same in-memory formats.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>