V4L/DVB (12510): soc-camera: (partially) convert to v4l2-(sub)dev API

Convert the soc-camera framework to use the v4l2-(sub)dev API. Start using
v4l2-subdev operations. Only a part of the interface between the
soc_camera core, soc_camera host drivers on one side and soc_camera device
drivers on the other side is replaced so far. The rest of the interface
will be replaced in incremental steps, and will require extensions and,
possibly, modifications to the v4l2-subdev code.

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