allwinner: Add a msgbox driver for use with SCPI

The function names follow the naming convention used by the existing
ARM SCPI client.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Change-Id: I543bae7d46e206eb405dbedfcf7aeba88a12ca48
diff --git a/plat/allwinner/common/allwinner-common.mk b/plat/allwinner/common/allwinner-common.mk
index 98bcf3e..83a7992 100644
--- a/plat/allwinner/common/allwinner-common.mk
+++ b/plat/allwinner/common/allwinner-common.mk
@@ -20,6 +20,7 @@
 				${AW_PLAT}/common/sunxi_common.c
 
 BL31_SOURCES		+=	drivers/allwinner/axp/common.c		\
+				drivers/allwinner/sunxi_msgbox.c	\
 				drivers/arm/gic/common/gic_common.c	\
 				drivers/arm/gic/v2/gicv2_helpers.c	\
 				drivers/arm/gic/v2/gicv2_main.c		\