iio: trigger: allow immutable triggers to be assigned

There are times when an assigned trigger to a device shouldn't ever
change after intialization.

Examples of this being used is when an provider device has a trigger
that is assigned to an ADC, which uses it populate data into a callback
buffer.

Signed-off-by: Matt Ranostay <matt@ranostay.consulting>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

include/linux/iio/trigger.h

@@ -131,6 +131,15 @@ int iio_trigger_register(struct iio_trigger *trig_info);
  **/
 void iio_trigger_unregister(struct iio_trigger *trig_info);
 
+/**
+ * iio_trigger_set_immutable() - set an immutable trigger on destination
+ *
+ * @indio_dev - IIO device structure containing the device
+ * @trig - trigger to assign to device
+ *
+ **/
+int iio_trigger_set_immutable(struct iio_dev *indio_dev, struct iio_trigger *trig);
+
 /**
  * iio_trigger_poll() - called on a trigger occurring
  * @trig:	trigger which occurred