ext: Add DRAMPower to enable on-line DRAM power modelling

This patch adds the open-source (BSD 3-clause) tool DRAMPower, commit
8d3cf4bbb10aa202d850ef5e5e3e4f53aa668fa6, to be built as a part of the
simulator. We have chosen this specific version of DRAMPower as it
provides the necessary functionality, and future updates will be
coordinated with the DRAMPower development team. The files added only
include the bits needed to build the library, thus excluding all
memory specifications, traces, and the stand-alone DRAMPower
command-line tool.

A future patch includes the DRAMPower functionality in the DRAM
controller, to enable on-line DRAM power modelling, and avoid using
post-processing of traces.