dt-bindings: iio: humidity: Add bindings for HTU21 and MS8607 sensors

Add device tree bindings for Measurement-Specialties HTU21 temperature & humidity sensor and humidity part of MS8607 sensor
Signed-off-by: NManivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: NJonathan Cameron <jic23@kernel.org>

Documentation/devicetree/bindings/iio/humidity/htu21.txt

+*HTU21 - Measurement-Specialties htu21 temperature & humidity sensor and humidity part of MS8607 sensor
+
+Required properties:
+
+	- compatible: should be "meas,htu21" or "meas,ms8607-humidity"
+	- reg: I2C address of the sensor
+
+Example:
+
+htu21@40 {
+	compatible = "meas,htu21";
+	reg = <0x40>;
+};