configs: Change fs_power.py to use absolute paths for stats

configs: Change fs_power.py to use absolute paths for stats

fs_power.py is an example script that demonstrates how power models
can be used with gem5. Previously, the formulas used to calculate the
dynamic and static power of the cores and the L2 cache were using
stats in equations as determined by their path relative to the
SimObject where the power model is attached to or full paths. This CL
changes these formulas to refer to the stats only by their full paths.

Change-Id: I91ea16c88c6a884fce90fd4cd2dfabcba4a1326c
Signed-off-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27893
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>