Align configuration with new SST change [1] requiring units for
memHierarchy's backend.mem_size parameter.
[1] c901abb4e7
Change-Id: I19fa09bec8aa453dc52d154598a4ebb20ea304d8
This patch adds an example configuration in ext/sst/tests/ that allows
an SST/gem5 instance to simulate a 4-core AArch64 system with SST's
memHierarchy components providing all the caches and memories.