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: NDuc Dang <dhdang@apm.com>
Signed-off-by: NTanmay Inamdar <tinamdar@apm.com>
Signed-off-by: NBjorn Helgaas <bhelgaas@google.com>