powerpc: autosize bpvo based on physical memory

Default moea64_bpvo_pool_size 327680 was insufficient for initial
memory mapping at boot time on systems with, for example, 64G and
no huge pages enabled.

Submitted by: Andre Silva <afscoelho@gmail.com>
Reviewed by: jhibbits, alfredo
Approved by: jhibbits (mentor)
Sponsored by: Eldorado Research Institute (eldorado.org.br)
Differential Revision: https://reviews.freebsd.org/D24102