iio: adc: mt6359: Move reference voltage to platform data

iio: adc: mt6359: Move reference voltage to platform data

In preparation to add support for new PMICs, add a `vref_mv`
member to struct mtk_pmic_auxadc_info and use it in place of
the AUXADC_VOLT_FULL definition.

As a consequence, the definition was also removed.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Link: https://patch.msgid.link/20250703141146.171431-5-angelogioacchino.delregno@collabora.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>