imx: support for i.MX8 SoCs misc IPC

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>
diff --git a/plat/imx/common/sci/sci_api.mk b/plat/imx/common/sci/sci_api.mk
index edaef4d..92c7190 100644
--- a/plat/imx/common/sci/sci_api.mk
+++ b/plat/imx/common/sci/sci_api.mk
@@ -9,4 +9,5 @@
 			plat/imx/common/sci/svc/pad/pad_rpc_clnt.c	\
 			plat/imx/common/sci/svc/pm/pm_rpc_clnt.c	\
 			plat/imx/common/sci/svc/rm/rm_rpc_clnt.c	\
-			plat/imx/common/sci/svc/timer/timer_rpc_clnt.c
+			plat/imx/common/sci/svc/timer/timer_rpc_clnt.c	\
+			plat/imx/common/sci/svc/misc/misc_rpc_clnt.c