board: lsxl: disable eth0

The board has only one network interface. The linux kernel will
gracefully skip a the ethernet interface if no connected PHY could be
probed. u-boot on the other hand will throw an error message. The kernel
device tree is about to be fixed. For now, just disable the ethernet
interface in our -u-boot.dtsi.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Stefan Roese <sr@denx.de>