mips: ralink: mt7621: fix phy-mode of external phy on GB-PC2

The phy-mode property must be defined on the MAC instead of the PHY. Define
phy-mode under gmac1 which the external phy is connected to.

Tested-by: Petr Louda <petr.louda@outlook.cz>
Signed-off-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Acked-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>