mb/lenovo/x220: Add support for ThinkLight

With this patch, the ThinkLight on the ThinkPad X220 can be controlled
through the OS.  This was initially done for the X201 in f63fbdb6:
mb/lenovo/x201: Add support for ThinkLight.

After applying this patch, the light can be controlled like this:

    echo on >/proc/acpi/ibm/light
    echo off >/proc/acpi/ibm/light

Or through sysfs at /sys/class/leds/tpacpi::thinklight

Unfortunately I do not own an X220 to test this.

Change-Id: Icead793694475e2f63353690203790ab7ce7c597
Signed-off-by: Stefan Ott <stefan@ott.net>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/40668
Reviewed-by: Alexander Couzens <lynxis@fe80.eu>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/lenovo/x220/dsdt.asl

@@ -35,4 +35,5 @@ DefinitionBlock(
 	}
 
 	#include <southbridge/intel/common/acpi/sleepstates.asl>
+	#include <ec/lenovo/h8/acpi/thinklight.asl>
 }