[media] V4L: soc-camera: call subdevice .s_power() method, when powering up or down

Currently soc-camera can use power regulators and platform specific
methods to power clients up and down. Additionally, client drivers can
provide their own subdevice .s_power() methods, acting directly on the
device. This patch adds calls to this method, when external power
supplies are on.

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