Add an 'aliases' property for ethernet device.

U-Boot performs a fix-up of the MAC address and will overwrite the values
from the Linux devicetree for aliased ethernet device. The MAC address can
be inherited from U-Boot by adding aliases of ethernet devices.
Signed-off-by: Kunihiko Hayashi <hayashi.kunihiko@socionext.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>