mem: Add DRAM power states to the controller

This patch adds power states to the controller. These states and the
transitions can be used together with the Micron power model. As a
more elaborate use-case, the transitions can be used to drive the
DRAMPower tool.

At the moment, the power-down modes are not used, and this patch
simply serves to capture the idle, auto refresh and active modes. The
patch adds a third state machine that interacts with the refresh state
machine.

src/mem/SConscript

@@ -81,6 +81,7 @@ CompoundFlag('Bus', ['BaseBus', 'BusAddrRanges', 'CoherentBus',
 DebugFlag('Bridge')
 DebugFlag('CommMonitor')
 DebugFlag('DRAM')
+DebugFlag('DRAMState')
 DebugFlag('LLSC')
 DebugFlag('MMU')
 DebugFlag('MemoryAccess')