power: Add a clock_period variable to power expressions

Currently 'Clock period in ticks' stat is not accessible in power equations .
This patch adds a variable 'clock_period' to be referenced to get the Clock period in
ticks

Signed-off-by: Sherif Elhabbal <elhabbalsherif@gmail.com>
Change-Id: Icac6a2e2003ed75d1680180e53343b0203139d72
Reviewed-on: https://gem5-review.googlesource.com/12664
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>

src/sim/power/mathexpr_powermodel.cc

@@ -127,7 +127,9 @@ MathExprPowerModel::getStatValue(const std::string &name) const
         return _temp;
     } else if (name == "voltage") {
         return clocked_object->voltage();
-    }
+    } else if (name=="clock_period") {
+        return clocked_object->clockPeriod();
+    } 
 
     // Try to cast the stat, only these are supported right now
     const auto it = stats_map.find(name);