doc: Complete the storage abstraction layer doc
Louis Mayencourt authored
Add uml sequence and class diagram to illustrate the behavior of the
storage abstraction layer.

Change-Id: I338262729f8034cc3d3eea1d0ce19cca973a91bb
Signed-off-by: default avatarLouis Mayencourt <louis.mayencourt@arm.com>
dbeace10