mem: Fix/Improve stats in src/mem

mem: Fix/Improve stats in src/mem

* Improved/Fixed the stat description of several stats.
* Fixed the value assigned to `warmupCycle` stat of cache tags,
it was set to curTick().
* Use ADD_STAT in CacheCmdStats.

Change-Id: Iabff7d42318fcc73df79ee7f3c5430f4c45555e5
Signed-off-by: Hoa Nguyen <hoanguyen@ucdavis.edu>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/39975
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>