mem-cache: Switch to new-style stats

mem-cache: Switch to new-style stats

This change puts cache and tag stats into a Stats::Group struct. This
makes it easier to identify stat updates (they are prefixed with
stat.) and adds hierarchy information for output formats that need it.

Change-Id: I2b8e9138f1cb977abb445ec864d80a79b588481d
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/21140
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>