arm64: dts: armada-3720-turris-mox: Add missing ethernet0 alias

U-Boot uses ethernet* aliases for setting MAC addresses. Therefore define
also alias for ethernet0.

Fixes: 7109d817 ("arm64: dts: marvell: add DTS for Turris Mox")
Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>