iio:event: Add timeout event info type

For adaptive threshold events, the current value is compared with a
(typically) low pass filtered version of the same signal that slowly
tracks large scale changes.  However, sometimes a step change can
result in a large lag before the low pass filtered version begins
to track the signal again.  Timeouts can be used to made an
instantaneous 'correction'.  Documentation of this attribute
is added in a later patch.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Link: https://lore.kernel.org/r/20210314181511.531414-11-jic23@kernel.org