config: Add low power sweep for DRAM

config: Add low power sweep for DRAM

This change adds a Python script to sweep a few parameters with a
goal to trigger transitions to the low power states of the DRAM
controller.

This script is largely based on the sweep.py but is helpful to study
the impact of inter-transaction delay on the behaviour of the DRAM
in addition to typical sweep params like stride size, bank
utilization and read percent. An idle period is added as the last
traffic generator state to target hitting self-refresh.

Change-Id: I34380afffbf2de9f4e997dfe9fff5e615e077524
Reviewed-by: Wendy Elsasser <wendy.elsasser@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/5725
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>