mvebu: uDPU: switch default kernel and U-Boot PHY mode

mvebu: uDPU: switch default kernel and U-Boot PHY mode

Certain SFP modules (most notably Nokia GPON ones) first check
connectivity on 1000base-x, and switch to 2500base-x afterwards. This
is considered a quirk so the phylink switches the interface to
2500base-x as well.

However, after power-cycling the uDPU device, network interface/SFP module
will not work correctly until the module is re-seated. This patch
resolves this issue by forcing the interface to be brought up in
2500base-x mode by default.

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