docs(spm): update threat model with FF-A v1.1

Update SPM's threat model to contain threats related to notifications
feature, compliant with FF-A v1.1 spec.

Change-Id: I4a825be5dd14137a0d04d532adfe5343714794c5
Signed-off-by: J-Alves <joao.alves@arm.com>
1 file changed