mem: Option to toggle DRAM low-power states

Adding an option to enable DRAM low-power states. The low power
states can have a significant impact on application performance
(sim_ticks) on the order of 2-3x, especially for compute-gpu apps.
The options allows for it to easily be enabled/disabled to compare
performance numbers. The option is disabled by default.

Change-Id: Ib9bddbb792a1a6a4afb5339003472ff8f00a5859
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/18548
Reviewed-by: Wendy Elsasser <wendy.elsasser@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>

configs/common/MemConfig.py

@@ -224,6 +224,11 @@ def config_mem(options, system):
             if issubclass(cls, m5.objects.DRAMCtrl) and opt_mem_ranks:
                 mem_ctrl.ranks_per_channel = opt_mem_ranks
 
+            # Enable low-power DRAM states if option is set
+            if issubclass(cls, m5.objects.DRAMCtrl):
+                mem_ctrl.enable_dram_powerdown = \
+                        options.enable_dram_powerdown
+
             if opt_elastic_trace_en:
                 mem_ctrl.latency = '1ns'
                 print("For elastic trace, over-riding Simple Memory "