powerpc: Scale intrcnt by mp_ncpus

On very large powerpc64 systems (2x22x4 power9) it's very easy to run out of
available IRQs and crash the system at boot. Scale the count by mp_ncpus,
similar to x86, so this doesn't happen. Further work can be done in the future
to scale the I/O IRQs as well, but that's left for the future.

Submitted by: mmacy
MFC after: 3 weeks