dt-bindings: iio: adc: adi,ad4080: add support for AD4081

Add device tree binding support for the AD4081 20-bit SAR ADC.
Add adi,ad4081 to the compatible enum.

A fallback compatible string to adi,ad4080 is not appropriate as the
AD4081 has a different LVDS CNV clock count maximum (2 vs 7), requiring
different driver configuration.

Acked-by: Conor Dooley <[e-mail]>
Signed-off-by: Antoniu Miclaus <[e-mail]>
Signed-off-by: Jonathan Cameron <[e-mail]>

Documentation/devicetree/bindings/iio/adc/adi,ad4080.yaml

···
   compatible:
     enum:
       - adi,ad4080
+      - adi,ad4081
       - adi,ad4084
 
   reg: