sim: Adding thermal model support

This patch adds basic thermal support to gem5. It models energy dissipation
through a circuital equivalent, which allows us to use RC networks.
This lays down the basic infrastructure to do so, but it does not "work" due
to the lack of power models. For now some hardcoded number is used as a PoC.
The solver is embedded in the patch.

src/sim/SubSystem.py

@@ -37,6 +37,7 @@
 #
 
 from m5.SimObject import SimObject
+from m5.params import *
 
 # An empty simobject. Used for organizing simobjects
 # into logical groups as subsystems of a larger
@@ -50,3 +51,7 @@ class SubSystem(SimObject):
     type = 'SubSystem'
     cxx_header = "sim/sub_system.hh"
     abstract = False
+
+    # Thermal doamin associated to this object, inheriting the parent's
+    # clock domain by default
+    thermal_domain = Param.ThermalDomain(NULL, "Thermal domain")