fujitsu-laptop: Add support for eco LED

There is an indicator LED signaling activated power saving mode
on certain Fujitsu laptop models. This has currently no use on Linux.
Export it to userspace.

Signed-off-by: Matej Groma <matejgroma@gmail.com>
Acked-by: Jonathan Woithe <jwoithe@just42.net>
Signed-off-by: Darren Hart <dvhart@linux.intel.com>