Support PEM that is not a PCI endpoint on ThunderX

Some chip revisions don't have their external PCIe buses
behind the internal bridge. Add support for FDT-configurable
PEMs but keep ability for PCIe enumeration.

Reviewed by: andrew, wma
Obtained from: Semihalf
Sponsored by: Cavium
Differential Revision: https://reviews.freebsd.org/D5285