arm64: renesas: salvator-x: Setup ethernet0 alias for U-Boot

Add an ethernet0 alias for the wired network card so that U-Boot can
find the device-node and add an appropriate "local-mac-address"
property.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>