lib.cdc: make domain properties private.

It is not correct to access domain properties from user code, because
it will not match the reality if DomainRenamer has been applied to
the module.

nmigen/compat/genlib/cdc.py

@@ -18,6 +18,7 @@ class MultiReg(NativeMultiReg):
                           .format(odomain, odomain),
                           DeprecationWarning, stacklevel=2)
         super().__init__(i, o, o_domain=odomain, n=n, reset=reset)
+        self.odomain = odomain
 
 
 @deprecated("instead of `migen.genlib.cdc.GrayCounter`, use `nmigen.lib.coding.GrayEncoder`")