docs(spm): add threat model for el3 spmc

Threat model for EL3 SPMC.
The mitigations are based on the guidance
provided in FF-A v1.1 EAC0 spec.

Signed-off-by: Shruti Gupta <shruti.gupta@arm.com>
Change-Id: I7f4c9370b6eefe6d1a7d1afac27e8b3a7b476072
4 files changed