arm: socfpga: socrates: Add eth0 alias to enable ethernet

arm: socfpga: socrates: Add eth0 alias to enable ethernet

This enables full ethernet usage, including U-Boot to write the board
specific MAC address (ethaddr) into the DT blob before passing it to
Linux.

Without this, the ethaddr is not detected in U-Boot at all, resulting
in this error upon bootup:

...
Model: EBV SOCrates
Net:
Error: ethernet@ff702000 address not set.
No ethernet found.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Marek Vasut <marex@denx.de>
Cc: Dinh Nguyen <dinguyen@opensource.altera.com>
Cc: Chin Liang See <clsee@altera.com>