wtm2: select write-combining memory for graphics

Auto-select marking the graphics memory as write-combining.

Change-Id: Icf61c5cbd129a97a106f0aaeca4e010d4799b4b8
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: http://review.coreboot.org/2981
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

src/mainboard/intel/wtm2/Kconfig

@@ -16,6 +16,7 @@ config BOARD_SPECIFIC_OPTIONS # dummy
 	select CHROMEOS
 	select EXTERNAL_MRC_BLOB
 	select CACHE_ROM
+	select MARK_GRAPHICS_MEM_WRCOMB
 
 config MAINBOARD_DIR
 	string