iio: adc: ti-ads1015: add threshold event support

The ADS1015 device provides programmable comparator that can issue an
interrupt on the ALERT pin.  This change adds the iio threshold event
support for that feature.

The ADS1015 device only have a single config register which contains an
input multiplexer selection, comparator settings, and etc.  So only a
single event channel can be enabled at a time.  Also enabling both buffer
and event are prohibited for simplicity.

Cc: Daniel Baluta <daniel.baluta@gmail.com>
Cc: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>