stdlib: Improving synthetic traffic generation.

This change adds a new traffic generator module to the standard
library that can read a .cfg file describing the traffic pattern
as a state machine. It wraps around the TrafficGen SimObject.
In addition it adds a method to ComplexGenerator to set the
traffic from outside using python generators like the example
found in configs/dram/sweep.py.

Change-Id: I5989bb900d26091e6e0e85ea63c741441b72069c
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/62473
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>
15 files changed