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/ruby/Ruby.py

@@ -130,6 +130,11 @@ def setup_memory_controllers(system, ruby, dir_cntrls, options):
             else:
                 mem_ctrl.port = dir_cntrl.memory
 
+            # Enable low-power DRAM states if option is set
+            if issubclass(MemConfig.get(options.mem_type), DRAMCtrl):
+                mem_ctrl.enable_dram_powerdown = \
+                        options.enable_dram_powerdown
+
         index += 1
         dir_cntrl.addr_ranges = dir_ranges