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>
diff --git a/src/mainboard/lenovo/t420/devicetree.cb b/src/mainboard/lenovo/t420/devicetree.cb
index 9f42cbb..809e3ee 100644
--- a/src/mainboard/lenovo/t420/devicetree.cb
+++ b/src/mainboard/lenovo/t420/devicetree.cb
@@ -36,6 +36,8 @@
 		end
 	end
 
+	register "pci_mmio_size" = "2048"
+
 	device domain 0 on
 		device pci 00.0 on
 			subsystemid 0x17aa 0x21ce