arm64: Increase dmap size to 95 TiB

The Ampere Altra has physical memory populated sparsely within the
physical address space. Increase the size of the dmap to cover all
physical memory.

Reviewed by: andrew
Approved by: scottl (implicit)
MFC after: 1 week
Sponsored by: Ampere Computing, Inc.
Differential Revision: https://reviews.freebsd.org/D26134