ACPI: thinkpad-acpi: add sysfs led class support for thinklight (v3.1)

Add a sysfs led class interface to the thinklight (light subdriver).

Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Cc: Richard Purdie <rpurdie@rpsys.net>
Signed-off-by: Len Brown <len.brown@intel.com>