stm32: Use MP_REGISTER_ROOT_POINTER().

This uses MP_REGISTER_ROOT_POINTER() to register all port-specific root
pointers in the stm32 port.

Signed-off-by: David Lechner <david@pybricks.com>

ports/stm32/timer.c

@@ -1684,3 +1684,5 @@ void timer_irq_handler(uint tim_id) {
         }
     }
 }
+
+MP_REGISTER_ROOT_POINTER(struct _pyb_timer_obj_t *pyb_timer_obj_all[MICROPY_HW_MAX_TIMER]);