dt-bindings: iio: amplifiers: Add docs for HMC425A Step Attenuator

Document support for Analog Devices MC425A Step Attenuator.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Beniamin Bia <beniamin.bia@analog.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>