iio: adc: mt6359: Add support for MediaTek MT6363 PMIC AUXADC

iio: adc: mt6359: Add support for MediaTek MT6363 PMIC AUXADC

MediaTek MT6363 is a PMIC found on MT8196/MT6991 board designs
and communicates with the SoC over SPMI.

This PMIC integrates an Auxiliary ADC (AUXADC) which has a grand
total of 54 ADC channels: 49 PMIC-internal channels, 2 external
NTC thermistor channels and 2 generic ADC channels (mapped to 7
PMIC ADC external inputs).

To use a generic ADC channel it is necessary to enable one of
the PMIC ADC inputs at a time and only then start the reading,
so in this case it is possible to read only one external input
for each generic ADC channel.

Due to the lack of documentation, this implementation supports
using only one generic ADC channel, hence supports reading only
one external input at a time.

Reviewed-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Link: https://patch.msgid.link/20250703141146.171431-6-angelogioacchino.delregno@collabora.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>