V4L/DVB (13648): soc-camera: add a private field to struct soc_camera_link

Up to now, if a client driver needed platform data apart from those contained
in struct soc_camera_link, it had to embed the struct into its own object. This
makes the use of such a driver in configurations other than soc-camera

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>