On the imx8mm is it possible to use shared memory between the A53s (Linux) and M4 (RTOS) without going through RPMsg? I don't see any notes stating that it's not possible, but also no examples of doing it without RPMsg.
You can implement the shared memory between A53 and M4 on your own by sharing the DDR between them. However, RPMSG does the same. So, I would suggest rather than implementing it, you use the supported RPMSG.