sim-power: Fix the way the power model accesses stats

sim-power: Fix the way the power model accesses stats

With the introduction of StatGroups the organization of stats has
changed and the power modeling framework has been broken. This CL uses
the new function Stats::resolve to retrieve pointers to the necesary
stats and use them in the power estimation formulas.

Change-Id: Iedaa97eeddf51f7a0a1f222918715da309943be3
Signed-off-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27892
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>