arm64: dts: Add APM X-Gene PCIe 64-bit prefetchable window

Add a large window (up to 64GB) for X-Gene PCIe nodes to support devices
that require huge BARs.

Each X-Gene PCIe node will now have two memory windows: a 32-bit
non-prefetchable window and a 64-bit prefetchable window.

[bhelgaas: changelog]
Signed-off-by: Duc Dang <dhdang@apm.com>
Signed-off-by: Tanmay Inamdar <tinamdar@apm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>