arm: mvebu: armada-3720-uDPU: fix PHY mode definition to sgmii-2500

Commit f49ac7e1 switched the default PHY speed to 3.125Gbit to resolve
issues with SFP modules. However, U-Boot does not have a "2500base-x"
phy-mode.

Resolve this by using "sgmii-2500" instead.

Signed-off-by: Jakov Petrina <jakov.petrina@sartura.hr>
Signed-off-by: Vladimir Vid <vladimir.vid@sartura.hr>
Reviewed-by: Stefan Roese <sr@denx.de>
Cc: Luka Perkov <luka.perkov@sartura.hr>
Cc: Stefan Roese <sr@denx.de>

arch/arm/dts/armada-3720-uDPU.dts

@@ -125,14 +125,14 @@
 &eth0 {
 	pinctrl-0 = <&pcie_pins>;
 	status = "okay";
-	phy-mode = "2500base-x";
+	phy-mode = "sgmii-2500";
 	managed = "in-band-status";
 	phy = <&ethphy0>;
 };
 
 &eth1 {
 	status = "okay";
-	phy-mode = "2500base-x";
+	phy-mode = "sgmii-2500";
 	managed = "in-band-status";
 	phy = <&ethphy1>;
 };