python: Avoid re-adding child when cloning SimObject

For SimObject type param, we should avoid duplicated addChild
call if it already belongs to other parent.

In the original implementation, the following code:

```

class A(SimObject):
  ...

class B(SimObject):
  a = Param.A(...)

class Top(RealView):
  a = A()
  b = B(a=a)
```

will generate incorrect warning:

```
warn: <orphan B>.a already has parent not resetting parent.
        Note: a is not a parameter of B
	warn: (Previously declared as <orphan Top>.a)
```

The code tries to add `a` as the child of `Top` as well as child of
`Top.b`, which is incorrect.

Change-Id: I8c55c5dd4cc0dd45c68169a2b08450ff053c07aa
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/60789
Reviewed-by: Yu-hsin Wang <yuhsingw@google.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>

src/python/m5/SimObject.py

@@ -697,7 +697,9 @@ class SimObject(object, metaclass=MetaSimObject):
         # are also param values get cloned properly.
         self._children = {}
         for key,val in ancestor._children.items():
-            self.add_child(key, val(_memo=memo_dict))
+            newval = val(_memo=memo_dict)
+            if not newval.has_parent():
+              self.add_child(key, newval)
 
         # Inherit parameter values from class using multidict so
         # individual value settings can be overridden but we still