[PowerPC] Make PPC 970 PMC SPRs the standard ones

And add a _74XX suffix to 74XX SPRs.

This is a preparation for adding support to POWER8/9 PMCs, which have most
SPRs equal to 970 ones.

Reviewed by: jhibbits
Sponsored by: Eldorado Research Institute (eldorado.org.br)
Differential Revision: https://reviews.freebsd.org/D26532