media: ti-vpe: cal: Register a media device

Enable the media controller API by registering a media device and
initializing the media entities corresponding to the video devices. The
context initialization is slightly refactored as a result. The media
graph will be built in a subsequent change.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Benoit Parrot <bparrot@ti.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>