V4L/DVB (12509): sh: prepare board-ap325rxa.c for v4l2-subdev conversion

We will be registering and unregistering the soc_camera_platform platform
device multiple times, therefore we need a .release() method and have to
nullify the kobj.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Acked-by: Paul Mundt <lethal@linux-sh.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>