arm64: dts: imx8mm-kontron: fix ethernet node name

The node name of Ethernet controller should be "ethernet" instead of
"usbether" as required by Ethernet controller devicetree schema:
 Documentation/devicetree/bindings/net/ethernet-controller.yaml

This patch can potentially affect boot loaders patching against full
node path instead of using device aliases.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Acked-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>