PPC64: cleanup APs startup routines

Cleaning up AP startup routines. This is a mix of changes
required to make PowerNV running and to modify the code
to be more robust. Previously, some races were seen if more
than 90CPUs were online.

Authored by: Patryk Duda <pdk@semihalf.com>
Submitted by: Wojciech Macek <wma@semihalf.com>
Obtained from: Semihalf
Sponsored by: IBM, QCM Technologies
Differential revision: https://reviews.freebsd.org/D14026