hwmon: (ntc_thermistor) Add DT with IIO support to NTC thermistor driver

This patch adds DT support to NTC driver to parse the
platform data.

Also adds the support to work as an iio device client.

During the probe ntc driver gets the respective channels of ADC
and uses iio_raw_read calls to get the ADC converted value.

Signed-off-by: Naveen Krishna Chatradhi <ch.naveen@samsung.com>
[Guenter Roeck: fixed Kconfig dependencies; use ERR_CAST]
Tested-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>