Follow these steps to get DRAMSim2 as part of gem5

1. Download DRAMSim2
    1.1 Go to ext/dramsim2 (this directory)
    1.2 Clone DRAMSim2: git clone git://github.com/dramninjasUMD/DRAMSim2.git

2. Compile gem5
    2.1 Business as usual

3. Run gem5 with DRAMSim2
    3.1 Use --mem-type=dramsim2 and set the device and system configuration