iio: adc: ab8500-gpadc: convert to device properties

iio: adc: ab8500-gpadc: convert to device properties

Make the conversion to firmware agnostic device properties. As part of
the conversion the IIO inkern interface 'of_xlate()' is also converted to
'fwnode_xlate()'. The goal is to completely drop 'of_xlate' and hence OF
dependencies from IIO.

Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20220715122903.332535-9-nuno.sa@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>