powerpc64: Add OPAL definitions

Summary:
Add additional OPAL PCI definitions and expand the code to use them in order to
ease the OPAL interface process for new comers.

These definitions came directly from the OPAL code and they are the same for
both PHB3 (POWER8) and PHB4 (POWER9).

Submitted by: Breno Leitao
Differential Revision: https://reviews.freebsd.org/D15432