Set the upper limit of the DMAP region to the limit of RAM as was found in
the physmap. This will reduce the likelihood of an issue where we have
device memory mapped in the DMAP. This can only happen if it is within the
same 1G block of normal memory.

Reviewed by: kib
Obtained from: ABT Systems Ltd
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D5938