Update the arm devmap code to also work with arm64.

There are a few differences between the two. On arm we need to provide a
list of addresses we may be mapping before we have initialised the virtual
memory subsystem, however on arm64 we allocate a small (2MiB for a 4k
granule) range to be used for such purposes.

Differential Revision: https://reviews.freebsd.org/D2249
Sponsored by: The FreeBSD Foundation