SPM: Add mailbox agent support in SFN model

Currently, the mailbox agent only supports IPC model.
This patch enables mailbox agent in SFN model. Unlike
IPC model, the reply request is handled as a sync one.
This patch covers the default dual-core routine and the
RSS routine.

Signed-off-by: Sherry Zhang <sherry.zhang2@arm.com>
Change-Id: I1035d81aa2427d2e58acaa187b44bbcef275426c
diff --git a/config/config_base.h b/config/config_base.h
index 811a42d..62f25b1 100644
--- a/config/config_base.h
+++ b/config/config_base.h
@@ -239,6 +239,11 @@
 #define PS_STACK_SIZE                          0x700
 #endif
 
+/* The stack size of the NS Agent Mailbox Secure Partition */
+#ifndef NS_AGENT_MAILBOX_STACK_SIZE
+#define NS_AGENT_MAILBOX_STACK_SIZE            0x800
+#endif
+
 /* SPM Partition Configs */
 
 #ifdef CONFIG_TFM_CONNECTION_POOL_ENABLE