dt-bindings: iio: frequency: change admv4420 maintainer

As the maintainer email no longer exists, change it to myself.
Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20221005085044.204701-2-nuno.sa@analog.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/devicetree/bindings/iio/frequency/adi,admv4420.yaml

@@ -7,7 +7,7 @@ $schema: http://devicetree.org/meta-schemas/core.yaml#
 title: ADMV4420 K Band Downconverter
 
 maintainers:
-  - Cristian Pop <cristian.pop@analog.com>
+  - Nuno Sá <nuno.sa@analog.com>
 
 description:
   The ADMV4420 is a highly integrated, double balanced, active