docs(threat-model): emphasize whether mitigations are implemented

For each threat, we now separate:
 - how to mitigate against it;
 - whether TF-A currently implements these mitigations.

A new "Mitigations implemented?" box is added to each threat to
provide the implementation status. For threats that are partially
mitigated from platform code, the original text is improved to make
these expectations clearer. The hope is that platform integrators will
have an easier time identifying what they need to carefully implement
in order to follow the security recommendations from the threat model.

Change-Id: I8473d75946daf6c91a0e15e61758c183603e195b
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>