If you’re using ST kernel, you need to use ST ATF, which doesn’t officially support this SoM and creates some kind of panic in the boot process. Regarding RPMSG drivers, do you mean kernel, bootloader or ATF?
I haven’t hit the DDR power init issue so I’m not too sure what’s happening. I’d recommend trying mainline kernel, ATF and boot with dts patches just to make sure your tools are working.