Implement bus_map_resource on arm64

This will allow us to allocate an unmapped memory resource, then
later map it with a specific memory attribute.

This is also needed for virtio with the modern PCI attachment.

Reviewed by: kib (via D29723)
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D29694

(cherry picked from commit fe3822497726ab84a1e3753be41e43e4d51aab0b)