iio: adc: ti-ads1015: add support for ADS1115 part

TI ADS1115 is a 16-bit resolution ADC that is register map
compatible with the ADS1015 device.

Signed-off-by: Matt Ranostay <matt.ranostay@intel.com>
Acked-by: Daniel Baluta <daniel.baluta@intel.com>
Acked-by: Crt Mori <cmo@melexis.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>