nb/intel/raminit (native): Read PCI mmio size from devicetree

Instead of hardcoding the PCI mmio size read it from devicetree.
Set a default value of 2048 MiB and 1024MiB for laptops without
discrete graphics.

Tested on Sandybridge Lenovo T520.

Change-Id: I791ebd6897c5ba4e2e18bd307d320568b1378a13
Signed-off-by: Patrick Rudolph <siro@das-labor.org>
Reviewed-on: https://review.coreboot.org/15140
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/mainboard/packardbell/ms2290/devicetree.cb

@@ -42,6 +42,8 @@ chip northbridge/intel/nehalem
 		end
 	end
 
+	register "pci_mmio_size" = "2048"
+
 	device domain 0 on
 		device pci 00.0 on # Host bridge
 			subsystemid 0x1025 0x0379