ti-adc081c: Initial triggered buffer support

Using this requires software triggers like CONFIG_IIO_HRTIMER_TRIGGER.

The device can be configured to do internal periodic sampling but does not
offer some sort of interrupt on data ready. Interrupts can only trigger when
values get out of a specific range.

Signed-off-by: Crestez Dan Leonard <leonard.crestez@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>