iio: adc: axp20x_adc: add support for AXP717 ADC

iio: adc: axp20x_adc: add support for AXP717 ADC

Add support for the AXP717 ADC. The AXP717 differs from other ADCs
in this series by utilizing a 14 bit ADC for all channels (a full 16
bits with the first 2 digits reserved). It also differs by lacking a
battery discharge current channel.

Note that while the current charge channel itself is included in this
driver for the AXP717 and listed in the datasheet, no scale or offset
was given for this channel. For now no scale or offset is provided in
this driver.

Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Chris Morgan <macromorgan@hotmail.com>
Link: https://patch.msgid.link/20240821215456.962564-13-macroalpha82@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>