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>

Documentation/devicetree/bindings/iio/resolver/ad2s90.txt

+Analog Devices AD2S90 Resolver-to-Digital Converter
+
+https://www.analog.com/en/products/ad2s90.html
+
+Required properties:
+  - compatible: should be "adi,ad2s90"
+  - reg: SPI chip select number for the device
+  - spi-max-frequency: set maximum clock frequency, must be 830000
+  - spi-cpol and spi-cpha:
+        Either SPI mode (0,0) or (1,1) must be used, so specify none or both of
+        spi-cpha, spi-cpol.
+
+See for more details:
+    Documentation/devicetree/bindings/spi/spi-bus.txt
+
+Note about max frequency:
+    Chip's max frequency, as specified in its datasheet, is 2Mhz. But a 600ns
+    delay is expected between the application of a logic LO to CS and the
+    application of SCLK, as also specified. And since the delay is not
+    implemented in the spi code, to satisfy it, SCLK's period should be at most
+    2 * 600ns, so the max frequency should be 1 / (2 * 6e-7), which gives
+    roughly 830000Hz.
+
+Example:
+resolver@0 {
+	compatible = "adi,ad2s90";
+	reg = <0>;
+	spi-max-frequency = <830000>;
+	spi-cpol;
+	spi-cpha;
+};