iio: adc: palmas: add support for iio threshold events

The palmas gpadc block has support for monitoring up to 2 ADC channels
and issue an interrupt if they reach past a set threshold. This change
hooks into the IIO events system and exposes to userspace the ability to
configure these threshold values for each channel, but only allow up to
2 such thresholds to be enabled at any given time. Trying to enable a
third channel will result in an error.

Userspace is expected to input calibrated, as opposed to raw, values as
threshold. However, it is not enough to do the opposite of what is done
when converting the other way around. To account for tolerances in the
ADC, the calculated raw threshold should be adjusted based on the ADC
specifications for the device. These specifications include the integral
nonlinearity (INL), offset, and gain error. To adjust the high
threshold, use the following equation:

  (calibrated value + INL) * Gain error + offset = maximum value  [1]

Likewise, use the following equation for the low threshold:

  (calibrated value - INL) * Gain error - offset = minimum value

The gain error is a combination of gain error, as listed in the
datasheet, and gain error drift due to temperature and supply. The exact
values for these specifications vary between palmas devices. This patch
sets the values found in TWL6035, TWL6037 datasheet.

[1] TI Application Report, SLIA087A, Guide to Using the GPADC in
    TPS65903x, TPS65917-Q1, TPS65919-Q1, and TPS65916 Devices.
Signed-off-by: Patrik Dahlström <risca@dalakolonin.se>
Link: https://lore.kernel.org/r/20230408114825.824505-9-risca@dalakolonin.seSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>