arm: dts: uDPU: switch default PHY speed to 3.125Gbit

This resolves issues with certain SFP modules.

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>