arm64: simplify physmap usage in pmap_bootstrap()

[mhorne]

Summary

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.

While here, use the raw value of physmap_idx.

No functional change intended.

[mhorne]

sys/arm64/arm64/pmap.c

1218

@markj your recent change d03e1ffbea6 coincided with mine, but I believe there is a minor bug here.

[markj]

sys/arm64/arm64/pmap.c

1218

Sigh, I missed this. :(

Would you commit this part on its own, with a "Fixes:" tag? Please consider it reviewed by me. If you don't have time, I can take care of it later today.

[mhorne]

Rebase after 32d2a1de3d64.