dt-bindings: net: dsa: Require ports or ethernet-ports

Bindings using dsa.yaml#/$defs/ethernet-ports specify that
a DSA switch node need to have a ports or ethernet-ports
subnode, and that is actually required, so add requirements
using oneOf.

Suggested-by: Rob Herring <[e-mail]>
Acked-by: Florian Fainelli <[e-mail]>
Reviewed-by: Krzysztof Kozlowski <[e-mail]>
Signed-off-by: Linus Walleij <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

Documentation/devicetree/bindings/net/dsa/dsa.yaml

···
             $ref: dsa-port.yaml#
             unevaluatedProperties: false
 
+oneOf:
+  - required:
+      - ports
+  - required:
+      - ethernet-ports
+
 ...