arm: dts: mediatek: convert gmac link mode to 2500base-x for r3

Ethernet on Bananapi-r3 is broken after

commit bd70f3ce ("net: mediatek: add support for SGMII 1Gbps auto-negotiation mode")

because changes from this commit were not applied to bpi-r3 devicetree too:

commit aef54ea1 ("arm: dts: medaitek: convert gmac link mode to 2500base-x")

Signed-off-by: Frank Wunderlich <frank-w@public-files.de>
Reviewed-by: Weijie Gao <weijie.gao@mediatek.com>