dt-bindings: iio: hmc425a: add entry for ADRF5740 Attenuator

The ADRF5740 is a silicon, 4-bit digital attenuator with 22 dB
attenuation control range in 2 dB steps.

Signed-off-by: Ana-Maria Cusco <ana-maria.cusco@analog.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20231113102535.51074-3-anamaria.cuscoo@gmail.com


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