[media] v4l: vsp1: Add V4L2_CID_ALPHA_COMPONENT control support

The control is used to configure the fixed alpha channel value, when
reading from memory in the RPF or writing to memory in the WPF.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>