dt: Document Holt HI-8435 bindings

Holt Integrated Circuits HI-8435 threshold detector bindings

Required properties:

 - compatible: should be "holt,hi8435"

 - reg: spi chip select number for the device

Recommended properties:

 - spi-max-frequency: definition as per

		Documentation/devicetree/bindings/spi/spi-bus.txt

Optional properties:

 - gpios: GPIO used for controlling the reset pin

Example:

sensor@0 {

	compatible = "holt,hi8435";

	reg = <0>;

	gpios = <&gpio6 1 0>;

	spi-max-frequency = <1000000>;

};