iio: potentiometer: Add support for the Renesas X9250 potentiometers

The Renesas X9250 integrates four digitally controlled potentiometers.
On each potentiometer, the X9250T has a 100 kOhms total resistance and
the X9250U has a 50 kOhms total resistance.

Signed-off-by: Herve Codina <herve.codina@bootlin.com>
Link: https://lore.kernel.org/r/20230509160852.158101-3-herve.codina@bootlin.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>