App: Add NS interface code moved from TF-M

Add NS interface code moved from TF-M. It includes OS wrapper headers
and RTX specific implementation of OS wrapper.

Signed-off-by: David Hu <david.hu@arm.com>
Change-Id: I0b88535eaa720b52d2090d5ed041987093df9d75
diff --git a/ns_interface/os_wrapper/tick.h b/ns_interface/os_wrapper/tick.h
new file mode 100644
index 0000000..b377b0e
--- /dev/null
+++ b/ns_interface/os_wrapper/tick.h
@@ -0,0 +1,28 @@
+/*
+ * Copyright (c) 2020, Arm Limited. All rights reserved.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ *
+ */
+
+#ifndef __OS_WRAPPER_TICK_H__
+#define __OS_WRAPPER_TICK_H__
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#include "common.h"
+
+/**
+ * \brief Return RTOS current tick count
+ *
+ * \return The current tick count
+ */
+uint32_t os_wrapper_get_tick(void);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* __OS_WRAPPER_TICK_H__ */