[nrf fromtree] Docs: Update ITS encryption graphic

The design of the ITS encryption was changed during the review
process but the graphic in the doc was not updated to the final design.
So update the graphic now.

Change-Id: I97c9ece25743bdd1eae05557d5e027b8128b4a90
Signed-off-by: Markus Swarowsky <[email protected]>
(cherry picked from commit d1c6dd1)
Signed-off-by: Markus Swarowsky <[email protected]>

Author: mswarowsky