doc: add security assessment of the storage SPs

Change the document structure and add a security assessment section.
Add the security assessment of the PSA ITS and PSA PS SPs.

Signed-off-by: Gyorgy Szing <gyorgy.szing@arm.com>
Change-Id: I642d39c14d30e6b7c97cfa5bd7f5c69327a2d5f8
4 files changed