Coalesce adjacent physical mappings.

This reduces the overhead when we have many small mappings, e.g. on some
EFI systems. This is to help use this code on arm64 where we may have a
large number of entries from the EFI firmware.

Obtained from: ABT Systems Ltd
Sponsored by: Turing Robotic Industries
Differential Revision: https://reviews.freebsd.org/D15477