Not all supported boards actually use the RK3399's built-in GMAC, while the
SoC TRM and the datasheet don't define some standard numbering in this case.
Thus, remove the ethernet0 alias from the RK3399 SoC dtsi file, and add the
same alias back to the appropriate board dts(i) files.

This is quite similar to the already performed migration of the mmcX aliases
from the Rockchip SoC dtsi files to the board dts(i) files.
Signed-off-by: Dragan Simic <dsimic@manjaro.org>
Link: https://lore.kernel.org/r/20879826c01fb9ead71c339866846ea794669802.1702366958.git.dsimic@manjaro.orgSigned-off-by: Heiko Stuebner <heiko@sntech.de>