[PowerPC64] add mpr to GENERIC64 and GENERIC64LE

Submitted by: Andre Fernando da Silva <andre.silva@eldorado.org.br>
Reviewed by: luporl, alfredo, Sreekanth Reddy <sreekanth.reddy@broadcom.com> (by email)
Sponsored by: Eldorado Research Institute (eldorado.org.br)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D25785