staging:iio:triggers:interrupt trigger - one per platform device.

Switching from one platform device registering a lot of triggers
to one for each trigger simplifies the code somewhat. It would be
relatively unusual to have more than a couple of such devices
registered so this change will not result in much additional overhead.

Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Reviewed-by: Lars-Peter Clausen <lars@metafoo.de>