arm: mvebu: armada-3720-uDPU.dts: Change back to phy-mode "2500base-x"

With commit 8678776d (arm: mvebu: armada-3720-uDPU: fix PHY mode
definition to sgmii-2500) the PHY mode was switch to "sgmii-2500", even
when this is functionally incorrect since "2500base-x" was not supported
in U-Boot at that time. As this mode is now supported (at least present
in the headers), this patch moves back to the orinal version.
Signed-off-by: NStefan Roese <sr@denx.de>
Cc: Jakov Petrina <jakov.petrina@sartura.hr>
Cc: Vladimir Vid <vladimir.vid@sartura.hr>
Cc: Luka Perkov <luka.perkov@sartura.hr>

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

@@ -126,14 +126,14 @@
 &eth0 {
 	pinctrl-0 = <&pcie_pins>;
 	status = "okay";
-	phy-mode = "sgmii-2500";
+	phy-mode = "2500base-x";
 	managed = "in-band-status";
 	phy = <&ethphy0>;
 };
 
 &eth1 {
 	status = "okay";
-	phy-mode = "sgmii-2500";
+	phy-mode = "2500base-x";
 	managed = "in-band-status";
 	phy = <&ethphy1>;
 };