docs(threat-model): cover threats inherent to receiving data over UART

TF-A supports reading input data from UART interfaces. This opens up
an attack vector for arbitrary data to be injected into TF-A, which is
not covered in the threat model right now.

Fill this gap by:

 - Updating the data flow diagrams. Data may flow from the UART into
   TF-A (and not only the other way around).

 - Documenting the threats inherent to reading untrusted data from a
   UART.

Change-Id: I508da5d2f7ad5d20717b958d76ab9337c5eca50f
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>
4 files changed