Python: Make the All proxy traverse SimObject children as well

This patch changes the behaviour of the All proxy parameter to not
only consider the direct children, but also do a pre-order depth-first
traversal of the object tree and append all results from the
children.

This is used in a later patch to find all the memories in the system,
independent of where they are located in the hierarchy.

src/python/m5/proxy.py

@@ -184,6 +184,8 @@ class AnyProxy(BaseProxy):
     def path(self):
         return 'any'
 
+# The AllProxy traverses the entire sub-tree (not only the children)
+# and adds all objects of a specific type
 class AllProxy(BaseProxy):
     def find(self, obj):
         return obj.find_all(self._pdesc.ptype)