cpu: Revamp saturating counters

cpu: Revamp saturating counters

Revamp the SatCounter class, improving comments, implementing
increment, decrement and read operators to solve an old todo,
and adding missing error checking.

Change-Id: Ia057c423c90652ebd966b6b91a3471b17800f933
Signed-off-by: Daniel <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/17992
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>