arm: dts: mediatek: Update mt7629 dts to reflect the latest dt-binding

* Removes mediatek,physpeed property from dtsi that is useless in PHYLINK
* Use the fixed-link property speed = <2500> to set the phy in 2.5Gbit.
* Set gmac1 to gmii mode that connect to a internal gphy

Signed-off-by: MarkLee <Mark-MC.Lee@mediatek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>