feat(mt8188): enable apusys mailbox mpu protect

Enable apusys mailbox mpu protect.

Change-Id: Idbf67084037b7ecf4926f57a901075f98540ee57
diff --git a/plat/mediatek/mt8188/include/platform_def.h b/plat/mediatek/mt8188/include/platform_def.h
index 43d12da..e72034c 100644
--- a/plat/mediatek/mt8188/include/platform_def.h
+++ b/plat/mediatek/mt8188/include/platform_def.h
@@ -31,6 +31,7 @@
 #define APU_RCX_CONFIG		(IO_PHYS + 0x09020000)
 #define APU_RCX_VCORE_CONFIG	(IO_PHYS + 0x090e0000)
 #define APU_MBOX0		(IO_PHYS + 0x090e1000)
+#define APU_MBOX1		(IO_PHYS + 0x090e2000)
 #define APU_RPCTOP		(IO_PHYS + 0x090f0000)
 #define APU_PCUTOP		(IO_PHYS + 0x090f1000)
 #define APU_AO_CTRL		(IO_PHYS + 0x090f2000)