arch-arm: Ensure counters keep events on checkpoint resume

arch-arm: Ensure counters keep events on checkpoint resume

Events were not being attached to counters after a checkpoint resume.
By not storing the enable private variable from the stored state the
recreation of the event to counter association is automatically carried.
The enable state is stored in the reg_pmcnten.

Change-Id: I46344df0882a9050c900efb2e8996d64dbfbf297
Reviewed-by: Sascha Bischoff <sascha.bischoff@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/5761
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>