dt-bindings: adc: axi-adc: update bindings for backend framework

'adi,adc-dev' is now deprecated and must not be used anymore. Hence,
also remove it from being required.

The reason why it's being deprecated is because the axi-adc CORE is now
an IIO service provider hardware (IIO backends) for consumers to make use
of. Before, the logic with 'adi,adc-dev' was the opposite (it was kind
of consumer referencing other nodes/devices) and that proved to be wrong
and to not scale.

Now, IIO consumers of this hardware are expected to reference it using the
io-backends property. Hence, the new '#io-backend-cells' is being added
so the device is easily identified as a provider.
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Nuno Sa <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20240210-iio-backend-v11-2-f5242a5fb42a@analog.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

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

@@ -39,12 +39,15 @@ properties:
     $ref: /schemas/types.yaml#/definitions/phandle
     description:
       A reference to a the actual ADC to which this FPGA ADC interfaces to.
+    deprecated: true
+
+  '#io-backend-cells':
+    const: 0
 
 required:
   - compatible
   - dmas
   - reg
-  - adi,adc-dev
 
 additionalProperties: false
 
@@ -55,7 +58,6 @@ examples:
         reg = <0x44a00000 0x10000>;
         dmas = <&rx_dma 0>;
         dma-names = "rx";
-
-        adi,adc-dev = <&spi_adc>;
+        #io-backend-cells = <0>;
     };
 ...