Documentation: media: camera-sensor: Move power management section

Move the power management section up, just after clocks, as it relates
to internal system resources and not features exposed to applications.
The text itself is otherwise unchanged.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>