[PPC64] Initial kernel minidump implementation

Based on POWER9BSD implementation, with all POWER9 specific code removed and
addition of new methods in PPC64 MMU interface, to isolate platform specific
code. Currently, the new methods are implemented on pseries and PowerNV
(D21643).

Reviewed by: jhibbits
Differential Revision: https://reviews.freebsd.org/D21551