docs(threat_model): cover the 'timing' side channel threat

Incorporate a timing side-channel attack into the TF-A generic
threat model. There is no software mitigation measures in TF-A
against this specific type of attack.

Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>
Change-Id: I10e53f8ed85a6da32de4fa6a210805f950018102
1 file changed