dt-bindings: iio: adc: add ad408x axi variant

Add a new compatible and related bindings for the fpga-based
AD408x AXI IP core, a variant of the generic AXI ADC IP.

The AXI AD408x IP is a very similar HDL (fpga) variant of the
generic AXI ADC IP, intended to control ad408x familiy.

Although there are some particularities added for extended
control of the ad408x devices such as the filter configuration.

Wildcard naming is used to match the naming of the published
firmware.

Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Antoniu Miclaus <antoniu.miclaus@analog.com>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://patch.msgid.link/20250516082630.8236-5-antoniu.miclaus@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/devicetree/bindings/iio/adc/adi,axi-adc.yaml

@@ -27,6 +27,7 @@ description: |
       the ad7606 family.
 
   https://wiki.analog.com/resources/fpga/docs/axi_adc_ip
+  https://analogdevicesinc.github.io/hdl/library/axi_ad408x/index.html
   https://analogdevicesinc.github.io/hdl/library/axi_ad485x/index.html
   http://analogdevicesinc.github.io/hdl/library/axi_ad7606x/index.html
 
@@ -34,6 +35,7 @@ properties:
   compatible:
     enum:
       - adi,axi-adc-10.0.a
+      - adi,axi-ad408x
       - adi,axi-ad7606x
       - adi,axi-ad485x