sim: Rename the root stats group to RootStats

sim: Rename the root stats group to RootStats

Currently, the name of the stats group of thr Root object is
Stats, which is likely to be confused with the Stats namespace.

This commit renames the struct to RootStats. This allows the
Stats namespace to be expressed as `Stats::`, which is
consistent with how the namespace is accessed in other part of
gem5.

Change-Id: Ieb425c3df1f5c0d5f11b1a467a36b2e0e07b2771
Signed-off-by: Hoa Nguyen <hoanguyen@ucdavis.edu>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/38915
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>