OBSOLETE!
External Xtensa TIE interface for the input queue type. Two callbacks should
be registered in order to send data to the object obj that
implements the interface, typically a processor. When the queue is accessed
the callbacks are called. user_object is the user object to use
in the empty_callback and data_callback callbacks.
The types of the empty_callback callback and data_callback are defined in
the xtensa/tie/cstub-extif.h file in the Xtensa model build
directory. The empty_callback should return 1 if the queue is empty,
otherwise 0. The data_callback is used for sending the data.