iio: Add a hysteresis event info attribute

For some devices it is possible to configure a hysteresis for threshold (or
similar) events. This patch adds a new hysteresis event info type which allows
for easy creation and read/write handling of the sysfs attribute.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
diff --git a/drivers/iio/industrialio-event.c b/drivers/iio/industrialio-event.c
index b7a5d7c..dac15b9 100644
--- a/drivers/iio/industrialio-event.c
+++ b/drivers/iio/industrialio-event.c
@@ -204,6 +204,7 @@
 static const char * const iio_ev_info_text[] = {
 	[IIO_EV_INFO_ENABLE] = "en",
 	[IIO_EV_INFO_VALUE] = "value",
+	[IIO_EV_INFO_HYSTERESIS] = "hysteresis",
 };
 
 static enum iio_event_direction iio_ev_attr_dir(struct iio_dev_attr *attr)