ARM: dts: sun8i: h3: Add ethernet0 alias to Beelink X2

Because "ethernet0" alias is missing, U-Boot doesn't generate board
specific MAC address. Effect of this is random MAC address every boot
and thus new IP address is assigned to the board.

Fix this by adding alias.

Fixes: 7389172f ("ARM: dts: sun8i: h3: Enable dwmac-sun8i on the Beelink X2")
Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
[Maxime: Removed unneeded comment]
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>