External Xtensa TIE interface for the import wire type. A callback should be
registered in order to send data to the object obj that
implements the interface, typically a processor. The callback is called when
the wire is accessed. The user_object is the user object to use
in the import_wire_func callback.
The type of the import_wire_func is defined in the
xtensa/tie/cstub-extif.h file in the Xtensa model build
directory.