Add an acpi attachment to the pci_host_generic driver and have the acpi
bus provide it with its needed memory resources.
This lets me use the external PCIe interface on the SoftIron Overdrive 3000,
and the internal (but not external) interface on the Cavium ThunderX. The
ThunderX external interface seems to be missing from ACPI tables on both
the development and production hardware I've looked at.