cpu: TraceGen fix for tick frequency check

Bug fix for check on protobuf file frequency being different than
global frequency.

The ASCII encoder script is also fixed, and the example trace used in
the regressions is updated.

cpu: Add support for protobuf input for the trace generator

This patch adds support for reading input traces encoded using
protobuf according to what is done in the CommMonitor.

A follow-up patch adds a Python script that can be used to convert the
previously used ASCII traces to protobuf equivalents. The appropriate
regression input is updated as part of this patch.

TrafficGen: Add a basic traffic generator regression

This patch adds a basic regression for the traffic generator. The
regression also serves as an example of the file formats used. More
complex regressions that make use of a DRAM controller model will
follow shortly.