media: rkvdec: Unstage the driver

The TODO list for unstaging being empty, the driver can now be moved to the
main media folder.

Also add myself as maintainer.

Signed-off-by: Detlev Casanova <detlev.casanova@collabora.com>
Reviewed-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Signed-off-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>