Add pmap_mapdev_attr for arm64
This is needed for efifb.
arm and ricv pmap (the two arch with arm64 that uses subr_devmap) have very
different implementation so for now only add this for arm64.
Tested with efifb on Pine64 with a few other patches.
Reviewed by: cognet
Differential Revision: https://reviews.freebsd.org/D15294