arm64: dts: imx8mn-ddr4-evk: Move iomuxc node to end of file

All nodes are better to follow alphabetical sort except iomuxc
which has huge pinctrl data, better to put it at the end of
file.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>