powerpc64: Set n_slbs = 32 for POWER9

Summary:
POWER9 also contains 32 slbs entries as explained by the POWER9 User Manual:

"For HPT translation, the POWER9 core contains a unified (combined for both

instruction and data), 32-entry, fully-associative SLB per thread"

Submitted by: Breno Leitao
Differential Revision: https://reviews.freebsd.org/D15128