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>

arch/mips/boot/dts/ralink/mt7621-gnubee-gb-pc2.dts

@@ -113,13 +113,13 @@ &pcie {
 
 &gmac1 {
 	status = "okay";
+	phy-mode = "rgmii-rxid";
 	phy-handle = <&ethphy5>;
 };
 
 &mdio {
 	ethphy5: ethernet-phy@5 {
 		reg = <5>;
-		phy-mode = "rgmii-rxid";
 	};
 };