Use EFI memory map to determine attributes for Acpi mappings on arm64.

AcpiOsMapMemory is used for device memory when e.g. an _INI method wants
to access physical memory, however, aarch64 pmap_mapbios is hardcoded to
writeback. Search for the correct memory type to use in pmap_mapbios.

Submitted by: Greg V <greg_unrelenting.technology>
Differential Revision: https://reviews.freebsd.org/D25201