Disable superpage use for stage 2 arm64 mappings

When modifying a stage 2 mapping we may need to call into the
hypervisor to invalidate the TLB. Until it is known if the cost of
this operation is less than the performance gains superpages offers
disable their use.

Reviewed by: kib. markj
Sponsored by: Innovate UK
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D37299