Dualcpu: Refine NS mailbox header files

- Move NS mailbox test structure and API into a dedicated header
  file tfm_ns_mailbox_test.h.
- Replace ns_mailbox_spin_lock()/unlock() with
  tfm_ns_mailbox_os_spin_lock()/unlock(), which are implemented by RTOS
  specific implementation. It decouples NS mailbox from CMSIS.

Change-Id: I22e8ce1df849529066429dda0640772a68161c60
Signed-off-by: David Hu <david.hu@arm.com>
diff --git a/cmake/install.cmake b/cmake/install.cmake
index d071b15..f7fa5bf 100644
--- a/cmake/install.cmake
+++ b/cmake/install.cmake
@@ -49,6 +49,7 @@
     install(FILES       ${INTERFACE_INC_DIR}/multi_core/tfm_multi_core_api.h
                         ${INTERFACE_INC_DIR}/multi_core/tfm_ns_mailbox.h
                         ${INTERFACE_INC_DIR}/multi_core/tfm_mailbox.h
+                        ${INTERFACE_INC_DIR}/multi_core/tfm_ns_mailbox_test.h
                         ${CMAKE_BINARY_DIR}/generated/interface/include/tfm_mailbox_config.h
             DESTINATION ${INSTALL_INTERFACE_INC_DIR})
 elseif (NOT TFM_PSA_API)