docs: Fix power model doxygen

Change-Id: I0a9a30bc4a89411e0f1dd897f0d1f05f20790b50
Signed-off-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-on: https://gem5-review.googlesource.com/9981
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>

src/doc/power_thermal_model.doxygen

@@ -126,3 +126,4 @@
   user to define a power model as an equation involving several statistics.
   There's also some automatic (or "magic") variables such as "temp", which
   reports temperature.
+*/

src/sim/power/power_model.hh

@@ -110,6 +110,8 @@ class PowerModelState : public SimObject
 };
 
 /**
+ * @sa \ref gem5PowerModel "gem5 Power Model"
+ *
  * A PowerModel is a class containing a power model for a SimObject.
  * The PM describes the power consumption for every power state.
  */

src/sim/power/thermal_model.hh

@@ -139,6 +139,8 @@ class ThermalReference : public SimObject, public ThermalEntity
 
 
 /**
+ * @sa \ref gem5PowerModel "gem5 Thermal Model"
+ *
  * A ThermalModel is the element which ties all thermal objects
  * together and provides the thermal solver to the system.
  * It is reponsible for updating temperature for all Thermal