mem-ruby: Update stats of AbstractController and derived classes

mem-ruby: Update stats of AbstractController and derived classes

This commit moves stats of AbstractController and its derived
classes to a Stats::Group struct.

Also, one of the controllers needs access to the ruby system profiler
stats, and Profiler's stats is now made public as a result.

Change-Id: Ibe04e33a6cf09b453564592d29293b354d0d33c9
Signed-off-by: Hoa Nguyen <hoanguyen@ucdavis.edu>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/38075
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>