sim: Add voltage() function to clocked_object

Adding voltage function which returns the current voltage
for a given clocked object. It's handy for power models and
similar stuff that need to retrieve voltage. Function
frequency() is already there, so I see no reason for not having
this one too.

src/sim/clocked_object.hh

@@ -212,6 +212,11 @@ class Clocked
         return clockDomain.clockPeriod();
     }
 
+    inline double voltage() const
+    {
+        return clockDomain.voltage();
+    }
+
     inline Cycles ticksToCycles(Tick t) const
     { return Cycles(divCeil(t, clockPeriod())); }