nanokernel: Add callback to _nano_timeout once again

It is now safe to introduce the callback since nano_timer_init now
calls _nano_timeout_init which does takes care of initializing all
the fields properly.

Change-Id: I5735eeebef233a0a541ec8b2a354b65da98082fc
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>