power: Fix regStats for PowerModel and PowerModelState

power: Fix regStats for PowerModel and PowerModelState

Every Stats::Group need to call the parent regStats to
make sure that the base Stats::Group::regStats() gets
called

JIRA: https://gem5.atlassian.net/projects/GEM5/issues/GEM5-319

Change-Id: I931941d8ec5f375f7e51e719d43ae31af095f661
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/26643
Tested-by: kokoro <noreply+kokoro@google.com>