arm64: dts: rockchip: add ethernet0 alias on rk3399

This is used by bootloaders to override the mac address in the devicetree
if needed.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>

arch/arm64/boot/dts/rockchip/rk3399.dtsi

@@ -56,6 +56,7 @@
 	#size-cells = <2>;
 
 	aliases {
+		ethernet0 = &gmac;
 		i2c0 = &i2c0;
 		i2c1 = &i2c1;
 		i2c2 = &i2c2;