dt-bindings: iio: humidity: add bindings for HDC100x sensors

Add the bindings for the family of HDC100x sensors.
Signed-off-by: NMichael Stecklein <m-stecklein@ti.com>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NJonathan Cameron <jic23@kernel.org>

Documentation/devicetree/bindings/iio/humidity/hdc100x.txt

+* HDC100x temperature + humidity sensors
+
+Required properties:
+  - compatible: Should contain one of the following:
+	ti,hdc1000
+	ti,hdc1008
+	ti,hdc1010
+	ti,hdc1050
+	ti,hdc1080
+  - reg: i2c address of the sensor
+
+Example:
+
+hdc100x@40 {
+	compatible = "ti,hdc1000";
+	reg = <0x40>;
+};