Add a way to map arm64 non-posted device memory

On arm64 we currently use a non-posted write for device memory, however
we should move to use posted writes. This is expected to work on most
hardware, however we will need to support a non-posted option for some
broken hardware.

Reviewed by: imp, manu, bcr (manpage)
Differential Revision: https://reviews.freebsd.org/D29722