mb/lenovo/x201: Add support for ThinkLight

The thinkpad-acpi driver uses the UCMS (CMOS) ACPI method to control the
ThinkLight from the Operating System. This patch adds partial support for
that method, enough to enable or disable the ThinkLight:

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

With the original BIOS the UCMS method exposes a wide range of values
through a generic /proc/acpi/ibm/cmos interface. With the changes suggested
in this patch that interface is also exposed but only accepts the commands
to enable or disable the ThinkLight; all other commands are ignored.

This change would potentially benefit all currently supported Thinkpad
models, I only have an X201 available for tests though.

Change-Id: I80285f6630b5830766d82e3ecd174c4a51aa9066
Signed-off-by: Stefan Ott <stefan@ott.net>
Reviewed-on: https://review.coreboot.org/19644
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>

src/ec/lenovo/h8/acpi/ec.asl

@@ -107,6 +107,11 @@ Device(EC)
 		Store(Arg0, USPW)
 	}
 
+	Method (LGHT, 1, NotSerialized)
+	{
+		Store(Arg0, KBLT)
+	}
+
 
 	/* Sleep Button pressed */
 	Method(_Q13, 0, NotSerialized)

src/mainboard/lenovo/x201/acpi/platform.asl

@@ -67,6 +67,21 @@ Method(_WAK,1)
 	Return(Package(){0,0})
 }
 
+Method(UCMS, 1, Serialized)
+{
+	Switch(ToInteger(Arg0))
+	{
+		Case (0x0c) /* Turn on ThinkLight */
+		{
+			\_SB.PCI0.LPCB.EC.LGHT(1)
+		}
+		Case (0x0d) /* Turn off ThinkLight */
+		{
+			\_SB.PCI0.LPCB.EC.LGHT(0)
+		}
+	}
+}
+
 /* System Bus */
 
 Scope(\_SB)