arm64: simplify physmap usage in pmap_bootstrap()

The subr_physmem.c facility provides guarantees about non-empty ranges.
Push this into pmap_bootstrap_dmap() and simplify; the lowest physical
memory range is always in the first item.

No functional change intended.

Reviewed by: markj
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D45759