doc: add slides concerning secure storage, fix layout
doc: add slides concerning secure storage, fix layout

Signed-off-by: Michael Brandl <46f1a0bd5592a2f9244ca321b129902a06b53e03@fineon.pw>
Acked-by: Jerome Forissier <ced78d268b4c9a0a20a59a9feeebd2ad90da03d3@linaro.org>

pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os