Docs: Rename folders for IPC and library model

As most of the files are named as xxx_func or xxx_ipc, rename the folder
to follow this default style, which makes the looking more aligned.
The library model also being mentioned as the function model, and the
IPC model is a PSA FF complied model. This explains why the suffix is
'psa' and 'func'.

Signed-off-by: Ken Liu <ken.liu@arm.com>
Change-Id: Id47e90624550ba038ed7b30bf494d88acde1b7be
diff --git a/docs/design_documents/source_structure.rst b/docs/design_documents/source_structure.rst
index 42641a1..826f6fc 100644
--- a/docs/design_documents/source_structure.rst
+++ b/docs/design_documents/source_structure.rst
@@ -147,8 +147,8 @@
 =================================== ===========================================
 include/\*                          SPM public headers.
 common/\*                           SPM common logic complies PSA-FF-M.
-psa_cmsis/\*                        CMSIS implementation for PSA-FF-M SPM. [1]
-func_cmsis/\*                       The library model implementation. [2]
+cmsis_psa/\*                        CMSIS implementation for PSA-FF-M SPM. [1]
+cmsis_func/\*                       The library model implementation. [2]
 \*                                  Implementation sources.
 =================================== ===========================================