asus-laptop: add led support

Add led support, using generic led class.  Thomas Tuttle's patch
<http://lkml.org/lkml/2006/7/6/247

> was very usefull.  We use
hotk->status to store led status because it's very hard to find
acpi method to get the right status...  To reduce the code,
I use a lot of macro (ASUS_LED, ASUS_LED_REGISTER, etc ...),
because the code is the same for all leds ...

Signed-off-by: Corentin Chary <corentincj@iksaif.net>
Signed-off-by: Len Brown <len.brown@intel.com>