SPM: Rename 'spm/common' into 'spm/ffm'

The name 'common' confused a bit, as its real purpose is for general
FF-M API implementation. Rename this folder as the first step for
re-structure patches.

There are some 'common' folder legacies still in the renamed 'ffm'
folder. They will be updated in the following patches.

Change-Id: Ic72fcf9ecf47f2185a82761edfdcbcd3c4569421
Signed-off-by: Ken Liu <Ken.Liu@arm.com>
diff --git a/docs/design_documents/source_structure.rst b/docs/design_documents/source_structure.rst
index 208c7bf..a549bf7 100644
--- a/docs/design_documents/source_structure.rst
+++ b/docs/design_documents/source_structure.rst
@@ -145,7 +145,7 @@
 Folder name                         Purpose
 =================================== ===========================================
 include/\*                          SPM public headers.
-common/\*                           SPM common logic complies with PSA-FF-M and
+ffm/\*                              SPM logic complies with PSA-FF-M and
                                     its necessary supporting functionalities,
                                     such as the runtime API and the thread
                                     operation, etc.