cpu: Turn BaseTrafficGen numSuppressed into a stat

This is changing numSuppressed from being a warn only variable into
a Stat so that it is visible at the end of simulation.

Change-Id: I934782e796c898bfc0e773cc88c597a68e403272
Reviewed-on: https://gem5-review.googlesource.com/11849
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>