arm64: Use the DMAP region in pmap_mapbios

As pmap_mapbios is used to map a physical address with a default memory
attribute we can try to use the DMAP region. For now check if all pages
are mapped, however it is expected this check can be removed later
after testing on more hardware.

Reviewed by: alc, markj, jhb (earlier version)
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D42732