V4L/DVB (9789): soc-camera: add a per-camera device host private data pointer

This pointer will be used by pxa_camera.c to point to its pixel format
data.

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