arm: dts: medaitek: convert gmac link mode to 2500base-x

Now that individual 2.5Gbps SGMII support has been added to
mtk-eth, all boards that use 2.5Gbps link with mt7531 must be
converted to use "2500base-x" instead of "sgmii".

Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>