ARM: dts: rockchip: Setup rk3288 ethernet0 alias for u-boot

Add an ethernet0 alias for the RK3288 mac interface so
that u-boot can find the device-node and fill in the mac address on
boards that support a wired network interface.

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>