media: ov8856: Add runtime PM callbacks

There were no runtime PM callbacks registered, leaving regulators being
enabled while the device is suspended on DT systems. Adjust and register
existing power controlling functions to turn them off/on.

Signed-off-by: Hidenori Kobayashi <hidenorik@chromium.org>
Reviewed-by: Tomasz Figa <tfiga@chromium.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>