docs(threat-model): add threat model for PSA FWU and TBBR FWU(recovery)

Added a threat model for PSA firmware update as well as TBBR FWU aka
firmware recovery.

Change-Id: I2396e13144076d7294f61f6817e1a8646225c6c2
Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>
2 files changed