dt-bindings:iio:resolver:adi,ad2s90: Conversion of binding to yaml.

Simple binding with a good description of why the spi-max-frequency is,
in practice not as high as the datasheet implies.  I've set the
maximum as per the value established in the description.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Matheus Tavares <matheus.bernardino@usp.br>
Cc: Alexandru Ardelean <alexandru.ardelean@analog.com>
Link: https://lore.kernel.org/r/20201031184854.745828-2-jic23@kernel.org