dt-bindings: iio: frequency: add adf4377 doc

Add device tree bindings for the ADF4377 driver.

Signed-off-by: Antoniu Miclaus <antoniu.miclaus@analog.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20221115110041.71495-1-antoniu.miclaus@analog.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>