powerpc/pmap: Add pmap_sync_icache() for radix pmap

DTrace pid provider writes to user space to set breakpoints. Failing to
sync the icache can lead to SIGTRAP. Radix pmap is the only one missing
a pmap_sync_icache() method, so the pid provider would only potentially
crash a process on a POWER9 or later system.

(cherry picked from commit 6f0b2a235a133a381634ba9a7f5f477c64db9873)