dt-bindings: net: dsa: mediatek,mt7530: add dsa-port definition for mt7988

dt-bindings: net: dsa: mediatek,mt7530: add dsa-port definition for mt7988

Add own dsa-port binding for SoC with internal switch where only phy-mode
'internal' is valid.

Signed-off-by: Frank Wunderlich <frank-w@public-files.de>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://patch.msgid.link/20250709111147.11843-6-linux@fw-web.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>