[media] media: soc-camera: update documentation

Update soc-camera documentation to reflect the current camera host API and
the use of the common V4L2 subdev API.

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