NXP's i.MX8 SoCs have system controller (M4 core) which takes
control of misc functions like temperature alarm, dma etc., other
Cortex-A clusters can send out command via MU (Message Unit) to
system controller for misc operation etc..

This patch adds misc IPC(inter-processor communication) support.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>