arm64: dts: imx8m: Add ddr controller nodes

This is used by the imx-ddrc devfreq driver to implement dynamic
frequency scaling of DRAM.

Support for proactive scaling via interconnect will come later. The
high-performance bus masters which need that (display, vpu, gpu) are
mostly not yet enabled in upstream anyway.

Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>