mb/lenovo/t400: Add support for ThinkLight

With this patch, the ThinkLight on the ThinkPad T400 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 T400 to test this.

Change-Id: I377854d6f54c5459e44626a7d7b61c513268183e
Signed-off-by: Stefan Ott <stefan@ott.net>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/40713
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Alexander Couzens <lynxis@fe80.eu>

src/mainboard/lenovo/t400/dsdt.asl

@@ -43,4 +43,6 @@ DefinitionBlock(
 
 	/* Dock support code */
 	#include "acpi/dock.asl"
+
+	#include <ec/lenovo/h8/acpi/thinklight.asl>
 }