media: rcar-vin: Add support for RGB formats with alpha component

The R-Car VIN module supports V4L2_PIX_FMT_ARGB555 and
V4L2_PIX_FMT_ABGR32 pixel formats. Add the hardware register setup and
allow the alpha component to be changed while streaming using the
V4L2_CID_ALPHA_COMPONENT control.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
[hverkuil-cisco@xs4all.nl: fix checkpatch warning for macro VNDMR_A8BIT]
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>