gem5: Addition of gem5 simulation platform

This commit adds a gem5 simulation platform. The platform is responsible for starting the gem5 simulation.

 Changes to be committed:
	modified:   devlib/__init__.py
	modified:   devlib/platform/__init__.py
	new file:   devlib/platform/gem5.py

devlib/platform/__init__.py

@@ -48,6 +48,11 @@ class Platform(object):
             self.name = self.model
         self._validate()
 
+    def setup(self, target):
+        # May be overwritten by subclasses to provide platform-specific
+        # setup procedures.
+        pass
+
     def _set_core_clusters_from_core_names(self):
         self.core_clusters = []
         clusters = []