iio: event: add optional event label support

iio: event: add optional event label support

This adds a new optional field to struct iio_info to allow drivers to
specify a label for the event. This is useful for cases where there are
many events or the event attribute name is not descriptive enough or
where an event doesn't have any other attributes.

The implementation is based on the existing label support for channels.
So either all events of a device have a label attribute or none do.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://lore.kernel.org/r/20231005-ad2s1210-mainline-v4-12-ec00746840fc@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>