Add an acpi attachment to the pci_host_generic driver and have the ACPI
bus provide it with its needed memory resources.

This allows us to use PCIe on the ThunderX2 and, with a previous version
of the patch, on the SoftIron 3000 with ACPI.

Obtained from: ABT Systems Ltd
Sponsored by: The FreeBSD Foundation
Sponsored by: DARPA, AFRL
Sponsored by: Cavium (Hardware)
Differential Revision: https://reviews.freebsd.org/D8767