iio: adc: max14001: New driver

iio: adc: max14001: New driver

The MAX14001/MAX14002 is configurable, isolated 10-bit ADCs for multi-range
binary inputs. In addition to ADC readings, the MAX14001/MAX14002 offers
more features, like a binary comparator, a filtered reading that can
provide the average of the last 2, 4, or 8 ADC readings, and an inrush
comparator that triggers the inrush current. There is also a fault feature
that can diagnose seven possible fault conditions. And an option to select
an external or internal ADC voltage reference.

MAX14001/MAX14002 features implemented so far:
- Raw ADC reading.
- MV fault disable.
- Selection of external or internal ADC voltage reference, depending on
whether it is declared in the device tree.

Co-developed-by: Kim Seer Paller <kimseer.paller@analog.com>
Signed-off-by: Kim Seer Paller <kimseer.paller@analog.com>
Signed-off-by: Marilene Andrade Garcia <marilene.agarcia@gmail.com>
Tested-by: Marcelo Schmitt <marcelo.schmitt1@gmail.com>
Reviewed-by: Marcelo Schmitt <marcelo.schmitt1@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>