ARM: dts: cygnus: add ethernet0 alias

In order to avoid Linux generating a random mac address on every boot,
add an ethernet0 alias that will allow u-boot to patch the dtb with
the MAC address.

Signed-off-by: Clément Péron <peron.clem@gmail.com>
Acked-by: Scott Branden <scott.branden@broadcom.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>