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>