ec/lenovo/h8: Add tablet mode switch method

thinkpad_acpi expects a MHKG method which returns the current
state of the tablet mode switch shifted left by 3. If such
method is not found, subsequent laptop/tablet mode events are
ignored.

Tested on a X220T.

Change-Id: Ic9ffea2ffe507b3692d1dd7411c52b813ec32146
Signed-off-by: Nicola Corna <nicola@corna.info>
Reviewed-on: https://review.coreboot.org/18328
Tested-by: build bot (Jenkins)
Reviewed-by: Alexander Couzens <lynxis@fe80.eu>

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

@@ -32,6 +32,9 @@ Device(EC)
 				HSPA, 1,
 		Offset (0x0C),
 				LEDS, 8,	/* LED state */
+		Offset (0x0F),
+				    , 7,
+				TBSW, 1,	/* Tablet mode switch */
 		Offset (0x1a),
 				DKR2, 1,	/* Dock register 2 */
 		Offset (0x2a),
@@ -354,6 +357,11 @@ Device(EC)
 		{
 			Return (0x07FFFFFF)
 		}
+		/* Report tablet mode switch state */
+		Method (MHKG, 0, NotSerialized)
+		{
+			Return (TBSW << 3)
+		}
 		/* Version */
 		Method (MHKV, 0, NotSerialized)
 		{