dt-bindings: iio: potentiometer: add max5432.yaml binding

Add a binding for the Maxim Integrated MAX5432-MAX5435 family of digital
potentiometers.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>