iio: adc: ti-ads1015: add indio_dev->dev.of_node reference

Add the pointer to the device tree node of the ADC so that iio
consumers can reference the respective channels.

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