powerpc: implement bus_map_resource and bus_unmap_resource DEVMETHODs

Implements bus_map_resource and bus_unmap_resource DEVMETHODs to be
used by powerpc targets. This is identical to the amd64 code.

Required by virtio-modern.

Reviewed by: bryanv
Sponsored by: Eldorado Research Institute (eldorado.org.br)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D28012