staging: iio: ad5933: add binding doc for ad5933

Add a devicetree documentation for the ad5933 and ad5934 impedance
converter, network analyzer.

Signed-off-by: Marcelo Schmitt <marcelo.schmitt1@gmail.com>
Signed-off-by: Gabriel Capella <gabriel@capella.pro>
Co-Developed-by: Gabriel Capella <gabriel@capella.pro>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>