thinkpad-acpi: enhance led support

Add support for extra LEDs on recent ThinkPads, and avoid registering
with the led class the LEDs which are not available for a given
ThinkPad model.

All non-restricted LEDs are always available through the procfs
interface, as the firmware doesn't care if an attempt is made to
access an invalid LED.

Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Signed-off-by: Len Brown <len.brown@intel.com>