arm64: dts: imx8mp: add "fsl,imx6sx-fec" compatible string

Add "fsl,imx6sx-fec" compatible string for fec node, then
i.MX8MP EVK ethernet function can work now.

Signed-off-by: Fugang Duan <fugang.duan@nxp.com>
Reviewed-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>