dt-bindings:iio:dac:microchip,mcp4725: fix properties for mcp4726

The vdd-supply property is optional if vref-supply is provided for
mcp4726.

Also the microchip,vref-buffered makes sense only if vref-supply is
specified.

Spotted by Jonathan during conversion to yaml.

Reported-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Tomas Novotny <tomas@novotny.cz>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20201216101316.1403-1-tomas@novotny.cz
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/devicetree/bindings/iio/dac/microchip,mcp4725.yaml

@@ -39,20 +39,39 @@ properties:
 
 allOf:
   - if:
-      not:
-        properties:
-          compatible:
-            contains:
-              const: microchip,mcp4726
+      properties:
+        compatible:
+          contains:
+            const: microchip,mcp4725
     then:
       properties:
         vref-supply: false
+      required:
+        - vdd-supply
+
+  - if:
+      properties:
+        compatible:
+          contains:
+            const: microchip,mcp4726
+    then:
+      anyOf:
+        - required:
+            - vdd-supply
+        - required:
+            - vref-supply
+
+  - if:
+      not:
+        required:
+          - vref-supply
+    then:
+      properties:
         microchip,vref-buffered: false
 
 required:
   - compatible
   - reg
-  - vdd-supply
 
 additionalProperties: false