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: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>