Increase the arm64 kernel address space to 512GB, and the DMAP region to
2TB. The latter can be increased in 512GB chunks by adjusting the lower
address, however more work will be needed to increase the former.

There is still some work needed to only create a DMAP region for the RAM
address space as on ARM architectures all mappings should have the same
memory attributes, and these will be different for device and normal memory.

Reviewed by: kib
Obtained from: ABT Systems Ltd
Relnotes: yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D5859