Clean up early arm64 pmap code

Early in the arm64 pmap code we need to translate between a virtual
address and a physical address. Rather than manually walking the page
table we can ask the hardware to do it for us.

Reviewed by: kib, markj
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D30357

(cherry picked from commit e779604f1d4e5fd0cdf3a9d1bb756b168f97b39c)