iio: adc: mt6359: Add support for MediaTek MT6373 PMIC AUXADC

iio: adc: mt6359: Add support for MediaTek MT6373 PMIC AUXADC

MediaTek MT6373 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 channels, of which usually only 9 are used as this
is usually paired with MT6363 on the same board.

For the Auxiliary ADC part, this reuses the same register layout
as the MT6363 PMIC, but exposes only a subset of the ADC chans.

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/20250703141146.171431-7-angelogioacchino.delregno@collabora.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>