mb/lenovo/x220: Increase MMIO space

With an external GPU connected via the expresscard slot this is
required.

Change-Id: I154721ff2c712cfe7eb79b8bf8943182c8c36548
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/51005
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Nico Huber <nico.h@gmx.de>

src/mainboard/lenovo/x220/devicetree.cb

@@ -27,7 +27,7 @@ chip northbridge/intel/sandybridge
 		end
 	end
 
-	register "pci_mmio_size" = "1024"
+	register "pci_mmio_size" = "2048"
 
 	device domain 0 on
 		subsystemid 0x17aa 0x21db inherit