dt: Document Holt HI-8435 bindings

These bindings can be used to register Holt HI-8435 threshold detector
Signed-off-by: Vladimir Barinov <vladimir.barinov@cogentembedded.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

Documentation/devicetree/bindings/iio/adc/hi8435.txt

+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>;
+};