dt-bindings: net: dsa: mediatek,mt7530: remove unnecessary dsa-port reference

dsa.yaml contains a reference to dsa-port.yaml, so a duplicate reference to
the binding isn't necessary. Remove this unnecessary reference.

Signed-off-by: Colin Foster <colin.foster@in-advantage.com>
Suggested-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>