mem,ext: Integrating DRAMSim3 with gem5

mem,ext: Integrating DRAMSim3 with gem5

Adding DRAMSim3 source code to the gem5 source code, the original
code was taken from umd-memsys github at https://github.com/umd-memsys/

Change-Id: I32c982206f33b0acf2121f322d15baa064c412c4
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/31757
Reviewed-by: Ayaz Akram <yazakram@ucdavis.edu>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>