dt-bindings:iio:humidity:htu21 Drop separate doc + add to trivial-devices

Very simple binding for this i2c device with no properties beyond
reg and compatible.  Hence doesn't need it's own document.
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20201031134110.724233-3-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>;
-};

Documentation/devicetree/bindings/trivial-devices.yaml

@@ -128,6 +128,10 @@ properties:
           - mcube,mc3230
             # MEMSIC 2-axis 8-bit digital accelerometer
           - memsic,mxc6225
+            # Measurement Specialities I2C temperature and humidity sensor
+          - meas,htu21
+            # Measurement Specialities temp and humidity part of ms8607 device
+          - meas,ms8607-humidity
             # Microchip differential I2C ADC, 1 Channel, 18 bit
           - microchip,mcp3421
             # Microchip differential I2C ADC, 2 Channel, 18 bit