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

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

With commit 8678776df6f5 (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: Stefan 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>