arm64: dts: imx8mq: Add ethernet alias

Add ethernet alias, so bootloader code can use this to find the
primary ethernet device, and set the MAC address.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>