dt-bindings: iio: potentiometer: Add the Renesas X9250 potentiometers

The Renesas X9250 is a quad digitally controlled potentiometers.

Signed-off-by: Herve Codina <herve.codina@bootlin.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230509160852.158101-2-herve.codina@bootlin.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>