mb/lenovo/t400: Change CMOS value name to Dual Graphics

To uniform the naming of the 'Dual Graphics' mode amongst the ThinkPads that
support it, the T400 CMOS value needs a change. This was the outcome of a
discussion on another patch: https://review.coreboot.org/#/c/coreboot/+/23040/

This might cause breakage for automated NVRAM configuration scripts, and
manuals. I only found one manual using the previous 'Switchable' option:
https://libreboot.org/docs/install/r400_external.html#a-note-about-gpus

Change-Id: I2e4d8bafbae5de97c78dab118f75fdefff1d7c37
Signed-off-by: Nico Rikken <nico@nicorikken.eu>
Reviewed-on: https://review.coreboot.org/27158
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>

src/mainboard/lenovo/t400/cmos.layout

@@ -128,7 +128,7 @@ enumerations
 11    12     352M
 12    0     Integrated Only
 12    1     Discrete Only
-12    2     Switchable
+12    2     Dual Graphics
 
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