The driver ignores the alpha component on output video nodes and
hardcodes the alpha component to 0 on capture video nodes. Make this
explicit by exposing XRGB formats.

Compatibility with existing userspace applications is handled by
selecting the XRGB format corresponding to the requested old RGB format.
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>