dt-bindings: iio: humidity: Add TI HDC20x0 support

Add device tree bindings for HDC2010/HDC2080 family of humidity and
temperature sensors.

Signed-off-by: Eugene Zaikonnikov <ez@norphonic.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>