ARM: mvebu: correct reference for "ethernet1" on DB-88F6820-AMC

The DB-88F6820-AMC connects ethernet@34000 and ethernet@70000 which are
labeled as eth2 and eth0 in armada-38x.dts. The ethernet@30000 (eth1) is
not used on the AMC board.

This eliminates the following bootup message

  Device 'ethernet@70000': seq 0 is in use by 'ethernet@34000'

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Signed-off-by: Stefan Roese <sr@denx.de>