Add Xilinx ZynqMP IPI mailbox service

Add IPI mailbox service to manage Xilinx ZynqMP IPI(Inter Processors
Interrupt) access.

Signed-off-by: Wendy Liang <jliang@xilinx.com>
diff --git a/plat/xilinx/zynqmp/platform.mk b/plat/xilinx/zynqmp/platform.mk
index a31e31d..bdd194b 100644
--- a/plat/xilinx/zynqmp/platform.mk
+++ b/plat/xilinx/zynqmp/platform.mk
@@ -42,7 +42,8 @@
 PLAT_INCLUDES		:=	-Iinclude/plat/arm/common/			\
 				-Iinclude/plat/arm/common/aarch64/		\
 				-Iplat/xilinx/zynqmp/include/			\
-				-Iplat/xilinx/zynqmp/pm_service/
+				-Iplat/xilinx/zynqmp/pm_service/		\
+				-Iplat/xilinx/zynqmp/ipi_mailbox_service/
 
 PLAT_BL_COMMON_SOURCES	:=	lib/xlat_tables/xlat_tables_common.c		\
 				lib/xlat_tables/aarch64/xlat_tables.c		\
@@ -75,4 +76,5 @@
 				plat/xilinx/zynqmp/pm_service/pm_svc_main.c	\
 				plat/xilinx/zynqmp/pm_service/pm_api_sys.c	\
 				plat/xilinx/zynqmp/pm_service/pm_ipi.c		\
-				plat/xilinx/zynqmp/pm_service/pm_client.c
+				plat/xilinx/zynqmp/pm_service/pm_client.c	\
+				plat/xilinx/zynqmp/ipi_mailbox_service/ipi_mailbox_svc.c