Interface: Clean up the tfm_api.h header file

This file does not look like an "interface" file as it contains
some SPM internal usage definitions.

This patch:
- Moves the SPM internal definitions to spm.h.
- Use the error codes in `internal_status_code.h` instead of the
  ones in tfm_api.h.
- Removes error codes in tfm_api.h.
- Renames tfm_api.h to tfm_veneers.h to reflect what it does.
- Corresponding changes in source files.

Change-Id: Iba5d1aae6837c9f0bf52c9a0c456023d19a735ea
Signed-off-by: Kevin Peng <kevin.peng@arm.com>
diff --git a/interface/include/tfm_ns_interface.h b/interface/include/tfm_ns_interface.h
index 6b55fc3..32a04c5 100644
--- a/interface/include/tfm_ns_interface.h
+++ b/interface/include/tfm_ns_interface.h
@@ -14,7 +14,7 @@
 #endif
 
 #include <stdint.h>
-#include "tfm_api.h"
+#include "tfm_veneers.h"
 
 typedef int32_t (*veneer_fn) (uint32_t arg0, uint32_t arg1,
                               uint32_t arg2, uint32_t arg3);