iio: adc: ina2xx: add INA236 support

iio: adc: ina2xx: add INA236 support

The calibration divisor is not directly specified in the datasheet, but can be calculated:

I = Current_LSB * Current
Current = ShuntVoltage * CAL / calibration_divisor
CAL = 0.00512 / (Current_LSB * Rshunt)
ShuntVoltage = Vshunt / ShuntVoltage_LSB

=> I = (0.00512 / (calibration_divisor*ShuntVoltage_LSB)) * (Vshunt / Rshunt)

Ohm's law, I = Vshunt / Rshunt
=> 0.00512 / (calibration_divisor*ShuntVoltage_LSB) = 1

ShuntVoltage_LSB = 2.5 uV = 0.0000025 V
=> calibration_divisor = 2048

Signed-off-by: Chuang Zhu <git@chuang.cz>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>