iio:adc:axp20x: add support for NTC thermistor

Most AXPxxx-based reference designs place a 10k NTC thermistor on a
TS pin. When appropriately configured, AXP PMICs will inject fixed
current (80uA by default) into TS pin and measure the voltage across a
thermistor. The PMIC itself will by default compare this voltage with
predefined thresholds  and disable battery charging whenever
the battery is too hot or too cold.

Alternatively, the TS pin can be configured as general-purpose
ADC input. This mode is not supported by the driver.

This patch allows reading the voltage on the TS pin. It can be then
either processed by userspace or used by kernel consumer like hwmon
ntc thermistor driver.

Signed-off-by: Evgeny Boger <boger@wirenboard.com>
Acked-by: Maxime Ripard <maxime@cerno.tech>
Reviewed-by: Quentin Schulz <foss+kernel@0leil.net>
Link: https://lore.kernel.org/r/20211118141233.247907-2-boger@wirenboard.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

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