Make opal_pci driver work with POWER9

Summary:
Coupled with r334365, this makes PCI work on POWER9. There is still more to
do to fully exploit the hardware capabilities, but this is sufficient to
enable USB and ethernet controllers on a POWER9 Talos II system.

Reviewed by: nwhitehorn, leitao
Differential Revision: https://reviews.freebsd.org/D15566