dt-bindings: iio: dac: ad5770r: Fix the file path

The following warning is seen with 'make dt_binding_check':

Documentation/devicetree/bindings/iio/dac/adi,ad5770r.yaml: $id: relative path/filename doesn't match actual path or filename

Fix it by removing the "bindings" directory from the file path.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>