EFI vt code for arm64
My branch is here : https://github.com/evadot/freebsd/tree/arm64_efifb
pmap_mapdev review : https://reviews.freebsd.org/D15294
Tested on Pine64 and Pinebook
Marking the PR in progress since code exists (and works) and some reviews are opened.
r335446 Add pmap_mapdev_attr for arm64
r336519 Raise the size of L3 table for early devmap on arm64
r336520 arm64: Add vt_efifb to GENERIC