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

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;

};