iio: adc: max1027: Introduce 12-bit devices support

Maxim's max12xx series is very similar to the max10xx series, with the
difference of the measurements depth which is upgraded from 10 to 12
bits per channel. Everything else looks the same.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>