dt-bindings:iio:resolver: Add docs for ad2s90

This patch adds the device tree binding documentation for the ad2s90
resolver-to-digital converter.

Signed-off-by: Matheus Tavares <matheus.bernardino@usp.br>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>