xpedite1k: Disable unused ethernet port 1

The XPedite1000 only has 2 available ethernet ports:
ppc_4xx_eth2 (EMAC2) and ppc_4xx_eth3 (EMAC3)

Signed-off-by: Peter Tyser <ptyser@xes-inc.com>
Signed-off-by: Stefan Roese <sr@denx.de>