mb/lenovo/t520: Add Dual Graphics CMOS support

Adds the CMOS option for Dual Graphics, as is present in more Lenovo models
already. Enabling this option ensures that the NVIDIA GPU is powered.

More PCI devices can be observed when activating this setting. It was verified
on a W520, also by loading a VGA option ROM and achieving a working Dual
Graphics system.

The CMOS default has been kept to 'Integrated Only', as the usage of Dual
Graphics requires an option ROM and drains the battery more quickly.

Change-Id: I41ccabd4554ca019684edd6f8b1c23679212c59f
Signed-off-by: Nico Rikken <nico@nicorikken.eu>
Reviewed-on: https://review.coreboot.org/26114
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Rudolph <siro@das-labor.org>

src/mainboard/lenovo/t520/cmos.layout

@@ -3,6 +3,7 @@
 ##
 ## Copyright (C) 2007-2008 coresystems GmbH
 ## Copyright (C) 2014 Vladimir Serbinenko
+## Copyright (C) 2018 Nico Rikken
 ##
 ## This program is free software; you can redistribute it and/or modify
 ## it under the terms of the GNU General Public License as published by
@@ -128,6 +129,7 @@ enumerations
 11    6	    224M
 12    0     Integrated Only
 12    1     Discrete Only
+12    2     Dual Graphics
 13    0     Disable
 13    1     AC and battery
 13    2     AC only