Add support for moving the DMAP range. This is needed as some AMD SoCs
place physical memory at an address outside the old DMAP range. This is an
issue as we rely on being able to move from PA -> VA using this range.

Obtained from: Patrick Wildt <patrick@bitrig.org> (earlier version)
Sponsored by: ABT Systems Ltd
Differential Revision: https://reviews.freebsd.org/D3885