board: sl28: move ethernet aliases to variant specific dtsi

board: sl28: move ethernet aliases to variant specific dtsi

The variants differ in their network configuration. Move the first two
network aliases to the proper variant device tree includes. This is in
prepartion for variant 1 and 2 support which has a different network
port mapping. The network aliases for the two internal ports will stay
in the common dtsi because they are present on all board variants.

This might leave a hole if there is no ethernet1 alias. This is
intended.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>