SPM: Add Asynchronous code to ns_agent_mailbox

Add AGENT_STATUS_IN_PROGRESS and ASYNC_MSG_REPLY defines.
Add handling of ASYNC_MSG_REPLY to ns_agent_mailbox main loop.
For now at least, this will never be called, this is simply to establish
the new API.

No behavioural change.

Change-Id: Iaee07d6eac097a20bf807844741cbecc54ad6ffd
Signed-off-by: Chris Brand <chris.brand@cypress.com>
diff --git a/secure_fw/include/async.h b/secure_fw/include/async.h
new file mode 100644
index 0000000..658b22e
--- /dev/null
+++ b/secure_fw/include/async.h
@@ -0,0 +1,24 @@
+/*
+ * Copyright (c) 2023 Cypress Semiconductor Corporation (an Infineon company)
+ * or an affiliate of Cypress Semiconductor Corporation. All rights reserved.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ *
+ */
+
+#ifndef __ASYNC_H__
+#define __ASYNC_H__
+
+#include "psa/error.h"
+
+/**
+ * Status returned for a request that will complete asynchronously.
+ */
+#define AGENT_STATUS_IN_PROGRESS  ((psa_status_t)-256)
+
+/**
+ * The signal number for the Secure Partition message acknowledgment.
+ */
+#define ASYNC_MSG_REPLY    (0x00000004u)
+
+#endif /* __ASYNC_H__ */