[PowerPC] hwpmc: add support for POWER8/9 PMCs

This change adds support for POWER8 and POWER9 PMCs (bare metal and
pseries).
All PowerISA 2.07B non-random events are supported.

Implementation was based on that of PPC970.

Reviewed by: jhibbits
Sponsored by: Eldorado Research Institute (eldorado.org.br)
Differential Revision: https://reviews.freebsd.org/D26110