PCI: xgene: Add support for a 64-bit prefetchable memory window

X-Gene PCIe controller has registers to support multiple memory ranges.

Add support for a 64-bit prefetchable memory 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>