arm: mvebu: turris_mox: Remove hardcoded ethernet node names

Armada 3720 DTS files in upstream kernel use ethernet nodes named
'ethernet@30000' and 'ethernet@40000'. U-Boot have them named 'neta@30000'
and 'neta@40000'. To have Turris Mox U-Boot board code independent of
ethernet node names, find ethernet node via alias.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>