[media] v4l: Add new alpha component control

The V4L2_CID_ALPHA_COMPONENT control is intended for the video capture
or memory-to-memory devices that are capable of setting up the per-pixel
alpha component to some arbitrary value. It allows to set the alpha
component for all pixels to an arbitrary value.

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>