Not all supported boards actually use the PX30's built-in (G)MAC, while the
SoC TRM and the datasheet don't define some standard numbering in this case.
Thus, remove the ethernet0 alias from the PX30 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/0d9da8959b4f567622676c34b5feb74c49489554.1702366958.git.dsimic@manjaro.orgSigned-off-by: Heiko Stuebner <heiko@sntech.de>