dt-bindings:iio:filter: add admv8818 doc

Add device tree bindings for the ADMV8818 Filter.

Signed-off-by: Antoniu Miclaus <antoniu.miclaus@analog.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>