iio: adc: Add TI ADS1015 ADC driver support

The driver has sysfs readings with runtime PM support for power saving.
It also offers buffer support that can be used together with IIO software
triggers.

Datasheet can be found here:
	http://www.ti.com.cn/cn/lit/ds/symlink/ads1015.pdf



Signed-off-by: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>