mem: Implement QoS Proportional Fair policy

Providing a configurable fair scheduling policy based on utilization;
utilization is directly proportional to a score which is inversely
proportional to the QoS priority. It is meant to avoid starvation of low
priority packets.

Users can tune the policy by adjusting the weight parameter (weight of
the following formula)

new_score = ((1.0 - weight) * old_score) + (weight * served_bytes)

Change-Id: I7679e234b916c57ebed06cec0ff3cff3cf2aef22
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/12359
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Anthony Gutierrez <anthony.gutierrez@amd.com>
Maintainer: Anthony Gutierrez <anthony.gutierrez@amd.com>