staging: iio: tsl2x7x: migrate *_thresh_period sysfs attributes to iio_event_spec

The sysfs attributes in_intensity0_thresh_period and
in_proximity0_thresh_period are currently directly created by the driver.
This patch migrates the creation of these sysfs attributes from the driver
to using the IIO core via iio_event_spec.

Signed-off-by: Brian Masney <masneyb@onstation.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>