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.