arm: mvebu: Don't include "netdev.h" in cpu.c

This is not needed any more since the switch to DM / DTS network
initialization on MVEBU. Lets remove it, as it otherwise leads
to compilation warning when CONFIG_NET is not enabled.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Luka Perkov <luka.perkov@sartura.hr>