misc: updated shabang for python script

The default python on MacOS doesn’t have an alias to python2.
The official python version supported in gem5 is Python2.7.

This patch updates the shabang according to the version required in gem5.

Change-Id: I9533c0f7858b5b3cab0ef101be1ee5cd718105b0
Reviewed-on: https://gem5-review.googlesource.com/c/15375
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>

util: Add script to plot DRAM low power sweep

This change adds a script to generate graphs from the stats file
output by the configuration script low_power_sweep.py.

The graphs show stacked bars for time spent and energy consumed
wherein each component of the stacked bar represents a DRAM power
state (Idle, Refresh, Active, Active Power-down, Precharge Power-down
and Self-refresh). The script generates one plot per delay value. It
also generates a pdf (--pdf option) in which the graphs are laid out
such that you can easily compare how the increasing delay and other
swept params affect the resulting energy.

Change-Id: Id80b0947bfde27e11e5505b23a3adb30f793a43f
Reviewed-by: Wendy Elsasser <wendy.elsasser@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/5727
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>