cpu: Make use of DRAMCtrl::AddrMap in the traffic generators

cpu: Make use of DRAMCtrl::AddrMap in the traffic generators

Use the enum defined in the memory controller rather than custom
strings and int that are later converted to the DRAMCtrl::AddrMap
enum.

Change-Id: Ie5b19f915f9990fd2b7505d4d1b17b6fc2100f9e
Signed-off-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/21080
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>