media: v4l: subdev: Document s_power() callback is deprecated

Runtime PM has been around for a decade or more, there's hardly a need to
use the V4L2 specific s_power() callback in drivers anymore. Document this
in s_power() callback documentation as well.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>