powerpc64: add Power8 and Power9 PMCs
Add support to allocate Power8 and 9 PMCs.
Submitted by: Leonardo Bianconi <leonardo.bianconi@eldorado.org.br>
Reviewed by: mhorne
Sponsored by: Instituto de Pesquisas Eldorado (eldorado.org.br)
Differential Revision: https://reviews.freebsd.org/D31109