powerpc: Blacklist the top 64kB range of the lower 4GB PA space

The PHB4 host bridge used by the POWER9 uses a 64kB range in 32-bit
space at the address 0xffff0000-0xffffffff. Reserve this range so that
DMA memory cannot be allocated within this range. This fixes seemingly
random crashes on a POWER9 system. Ideally this range will have been
reserved by the firmware, but as of now this is not the case.

Submitted by: git_bdragon.rtk0.net
Reviewed by: nwhitehorn
Approved by: re(kib)
Differential Revision: https://reviews.freebsd.org/D17183