hwmon: (ntc_thermistor): try reading processed

hwmon: (ntc_thermistor): try reading processed

Before trying the custom method of reading the sensor
as raw and then converting, we want to use
iio_read_channel_processed_scale() which first tries to
see if the ADC can provide a processed value directly,
else reads raw and applies scaling inside of IIO
using the scale attributes of the ADC. We need to
multiply the scaled value with 1000 to get to
microvolts from millivolts which is what processed
IIO channels returns.

Keep the code that assumes 12bit ADC around as a
fallback.

This gives correct readings on the AB8500 thermistor
inputs used in the Ux500 HREFP520 platform for reading
battery and board temperature.

Cc: Peter Rosin <peda@axentia.se>
Cc: Chris Lesiak <chris.lesiak@licor.com>
Cc: Jonathan Cameron <jic23@kernel.org>
Cc: linux-iio@vger.kernel.org
Link: https://lore.kernel.org/linux-iio/20201224011607.1059534-1-linus.walleij@linaro.org/
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Tested-by: Chris Lesiak <chris.lesiak@licor.com>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20210308100219.2732156-2-linus.walleij@linaro.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>