powerpc_mmu_radix: add leaf page for wired mappings when pmap_enter(psind=1)

This applies the fix to powerpc's pmap as was done in commit aa3bcaad51076ceb
and d0941ed9b5c39d92 for amd64 and riscv pmaps, respectively.

Reported by: Jenkins
Reviewed by: bojan.novkovic_fer.hr, markj
Fixes: e4078494f344bcba8709216bd601efa3dd05f6b3
Differential Revision: https://reviews.freebsd.org/D43339