Add phy-mode property required by phylink on gmac2

Fixes: b8fc9f30 ("net: ethernet: mediatek: Add basic PHYLINK support")
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Link: https://lore.kernel.org/r/70e3eff31ecd500ed4862d9de28325a4dbd15105.1583648927.git.sean.wang@mediatek.comSigned-off-by: Matthias Brugger <matthias.bgg@gmail.com>