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

The common dsa-port.yaml does this (and more) since commit 2ec2fb83
("dt-bindings: net: dsa: make phylink bindings required for CPU/DSA
ports").

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>