iio: adc: meson-saradc: squash and share the common adc platform data

Extract and promote common adc platform data into a new structure,
to make it better share the info between several SoCs,
this will avoid duplicating the code all over the place,
Save a few memory and make the code more maintainable.

Signed-off-by: Yixun Lan <yixun.lan@amlogic.com>
Acked-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>