media: ti-vpe: cal: Create subdev for CAMERARX

Create and register V4L2 subdevs for the CAMERARX instances, and link
them in the media graph to the sensors and video nodes. The subdev API
is not exposed to userspace at this point, and no subdev operation is
implemented, but the media controller graph is visible to applications.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>