V4L/DVB: soc-camera: add runtime pm support for subdevices

To save power soc-camera powers subdevices down, when they are not in use,
if this is supported by the platform. However, the V4L standard dictates,
that video nodes shall preserve configuration between uses. This requires
runtime power management, which is implemented by this patch. It allows
subdevice drivers to specify their runtime power-management methods, by
assigning a type to the video device.

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