V4L/DVB (11705): soc-camera: prepare for the platform driver conversion

Add a platform driver to soc_camera.c. This way we preserve backwards
compatibility with existing platforms and can start converting them one by one
to the new platform-device soc-camera interface.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>