dt-bindings: net: dsa: mt7530: stop requiring phy-mode on CPU ports

The common dsa-port.yaml does this (and more) since commit 2ec2fb8331af
("dt-bindings: net: dsa: make phylink bindings required for CPU/DSA
ports").

Signed-off-by: Vladimir Oltean <[e-mail]>
Reviewed-by: Florian Fainelli <[e-mail]>
Reviewed-by: Arınç ÜNAL <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Signed-off-by: Jakub Kicinski <[e-mail]>

Documentation/devicetree/bindings/net/dsa/mediatek,mt7530.yaml

···
           - if:
               required: [ ethernet ]
             then:
-              required:
-                - phy-mode
-
               properties:
                 reg:
                   enum: