mem: Fill the new packet ID fields with master IDs when tracing packets.

mem: Fill the new packet ID fields with master IDs when tracing packets.

This will let somebody consuming the memory packet trace make sense out of
the master IDs passed along with individual accesses.

Change-Id: I621d915f218728066ce95e6fc81f36d14ae7e597
Reviewed-on: https://gem5-review.googlesource.com/4800
Reviewed-by: Rahul Thakur <rjthakur@google.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>