dt-bindings: iio: frequency: Add docs for ADF4371 PLL

Document support for Analog Devices ADF4371 SPI Wideband Synthesizer.

Signed-off-by: Stefan Popa <stefan.popa@analog.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>