iio: adc: Add support for MediaTek MT6357/8/9 Auxiliary ADC

iio: adc: Add support for MediaTek MT6357/8/9 Auxiliary ADC

Add a driver to support reading the Auxiliary ADC IP found in the
MediaTek MT6357, MT6358 and MT6359 Power Management ICs, featuring
a different register layout, configuration reset and ADC reading
sequence from the other already supported MediaTek SoC or PMIC
(aux)ADC HW.

This driver provides multiple ADC channels for system monitoring,
such as battery voltage, PMIC temperature, PMIC-internal voltage
regulators temperature, and others.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Link: https://patch.msgid.link/20240604123008.327424-5-angelogioacchino.delregno@collabora.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>