ACPI: Document how to refer to LEDs from remote nodes

Document referring to LEDs from remote device nodes, such as from camera
sensors.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>