configs: use SimpleMemory when using ruby in se mode

configs: use SimpleMemory when using ruby in se mode
A recent changeset altered the default memory class to DRAMCtrl.  In se mode,
ruby uses the physical memory to check if a given address is within the bounds
of the physical memory.  SimpleMemory is enough for this.  Moreover,
SimpleMemory does not check whether it is connected or not, something which
DRAMCtrl does.