dt-bindings: iio: amplifiers: ad8366: add adrf5702/3 support

dt-bindings: iio: amplifiers: ad8366: add adrf5702/3 support

Add compatible entries for ADRF5702 and ADRF5703 Digital Attenuators.
ADRF5702 is an 8-bit DSA with a step of 0.125 dB and ADRF5703 is a 7-bit
DSA with a step 0.25 dB. Then, each device ends up with its own gain
range, hence no fallback compatibles are used.

Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Signed-off-by: Rodrigo Alencar <rodrigo.alencar@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>