mem: Trace the request master ID in the MemTraceProbe.

mem: Trace the request master ID in the MemTraceProbe.

There's a spot for it in the packet trace protobuf, so we should fill it
with something.

Change-Id: I784feb3f668e1b20d67b6ef98d012bcf59b7bd40
Reviewed-on: https://soc-sim-internal-review.googlesource.com/3483
Reviewed-by: Rahul Thakur <rjthakur@google.com>
Reviewed-on: https://gem5-review.googlesource.com/4781
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>