sim: Make the drain state a global typed enum

The drain state enum is currently a part of the Drainable
interface. The same state machine will be used by the DrainManager to
identify the global state of the simulator. Make the drain state a
global typed enum to better cater for this usage scenario.

src/mem/dram_ctrl.cc

@@ -2189,9 +2189,9 @@ DRAMCtrl::drain(DrainManager *dm)
     }
 
     if (count)
-        setDrainState(Drainable::Draining);
+        setDrainState(DrainState::Draining);
     else
-        setDrainState(Drainable::Drained);
+        setDrainState(DrainState::Drained);
     return count;
 }