mb/lenovo/x200: Add support for ThinkLight

With this patch, the ThinkLight on the ThinkPad X200 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

I have tested it on an X200 with Kernel 5.4 and it seems to work fine.

Change-Id: I14752ab33484122248959517e73f96b6783b1f65
Signed-off-by: Stefan Ott <stefan@ott.net>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/40620
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/mainboard/lenovo/x200/dsdt.asl

@@ -41,4 +41,6 @@ DefinitionBlock(
 
 	/* Dock support code */
 	#include "acpi/dock.asl"
+
+	#include <ec/lenovo/h8/acpi/thinklight.asl>
 }