Documentation: dt: iio: humidity: add hts221 sensor device binding

* HTS221 STM humidity + temperature sensor

Required properties:

- compatible: should be "st,hts221"

- reg: i2c address of the sensor / spi cs line

Optional properties:

- interrupt-parent: should be the phandle for the interrupt controller

- interrupts: interrupt mapping for IRQ. It should be configured with

  flags IRQ_TYPE_LEVEL_HIGH or IRQ_TYPE_EDGE_RISING.

  Refer to interrupt-controller/interrupts.txt for generic interrupt

  client node bindings.

Example:

hts221@5f {

	compatible = "st,hts221";

	reg = <0x5f>;

	interrupt-parent = <&gpio0>;

	interrupts = <0 IRQ_TYPE_EDGE_RISING>;

};