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/src/tfm_platform_api.c b/interface/src/tfm_platform_api.c
index d709607..4fc564b 100644
--- a/interface/src/tfm_platform_api.c
+++ b/interface/src/tfm_platform_api.c
@@ -7,6 +7,7 @@
 
 #include <stdbool.h>
 #include "tfm_platform_api.h"
+#include "psa/client.h"
 #include "psa_manifest/sid.h"
 
 enum tfm_platform_err_t tfm_platform_system_reset(void)