mem: Use address range to find the right physical address

mem: Use address range to find the right physical address

Previously, we used the start address to determine the right physical
memory while servicing memory requests. This change uses the full
address range to correctly determine the right physical memory and
expose bugs where requests might not fully map to a single physical
memory.

Change-Id: I183d7552918106000f917a62ceb877511ff0ff71
Reviewed-on: https://gem5-review.googlesource.com/11118
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>