mb/lenovo/t420: Add support for ThinkLight

With this patch, the ThinkLight on the ThinkPad T420 can be controlled
through the OS.  The same change was done for the ThinkPad X200 in
b45912f4: mb/lenovo/x200: 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 a T420 to test this.

Change-Id: I4f9a9937a45995b72a9712919316e95bb8f82f45
Signed-off-by: Stefan Ott <stefan@ott.net>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/40714
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/mainboard/lenovo/t420/dsdt.asl

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