dt-bindings: iio: potentiometer: Add Maxim DS3502 in trivial-devices

Maxim DS3502 is a 7 bit nonvolatile digital potentiometer.
Add DS3502 binding into trivial-devices.yaml.

Signed-off-by: Jagath Jog J <jagathjog1996@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20220223163525.13399-8-jagathjog1996@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>