sim: Updated ClockedObject power state warning

sim: Updated ClockedObject power state warning

To prevent this warning from printing for *every* simulation, this patch
adds a check to only print the warning if we are not at the beginning of
simulation.

Change-Id: I7f6154f0ca26bef6280f909f799aa1c7936b624a
Signed-off-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-on: https://gem5-review.googlesource.com/3840
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>