Update the virtio driver to work on the ARM AArch64 Foundation Model.
There are two main parts to get it to work, 1) most of the register
accesses need to be word sized, other than the config register which
needs to be byte aligned, and 2) we don't need the platform driver
for this to work on the Foundation Model, allow it to be NULL.

Differential Revision: https://reviews.freebsd.org/D1240
Reviewed by: bryanv
Sponsored by: The FreeBSD Foundation