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/apple/macbookair4_2/devicetree.cb b/src/mainboard/apple/macbookair4_2/devicetree.cb
index a43b9f2..226ec04 100644
--- a/src/mainboard/apple/macbookair4_2/devicetree.cb
+++ b/src/mainboard/apple/macbookair4_2/devicetree.cb
@@ -30,6 +30,9 @@
 			end
 		end
 	end
+
+	register "pci_mmio_size" = "2048"
+
 	device domain 0x0 on
 		chip southbridge/intel/bd82x6x # Intel Series 6 Cougar Point PCH
 			register "c2_latency" = "0x0065"