docs(threat-model): revamp threat #9

docs(threat-model): revamp threat #9

Reword the description of threat #9 to make it more future-proof for
Arm CCA. By avoiding specific references to secure or non-secure
contexts, in favour of "worlds" and "security contexts", we make the
description equally applicable to 2-world and 4-world architectures.

Note that there are other threats that would benefit from such a
similar revamp but this is out of scope of this patch.

Also list malicious secure world code as a potential threat
agent. This seems to be an oversight in the first version of the
threat model (i.e. this change is not related to Arm CCA).

Change-Id: Id8c8424b0a801104c4f3dc70e344ee702d2b259a
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>