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/samsung/lumpy/devicetree.cb

@@ -34,6 +34,8 @@ chip northbridge/intel/sandybridge
 		end
 	end
 
+	register "pci_mmio_size" = "1024"
+
 	device domain 0 on
 		ioapic_irq 4 INTA 0x10
 		ioapic_irq 4 INTB 0x11

src/mainboard/samsung/stumpy/devicetree.cb

@@ -32,6 +32,8 @@ chip northbridge/intel/sandybridge
 		end
 	end
 
+	register "pci_mmio_size" = "1024"
+
 	device domain 0 on
 		subsystemid 0x1ae0 0xc000 inherit
 		device pci 00.0 on end # host bridge