docs(threat-model): supply chain threat model TF-A

Software supply chain attacks aim to inject malicious code into a
software product. There are several ways a malicious code can be
injected into a software product (open-source project).

These include:
- Malicious code commits
- Malicious dependencies
- Malicious toolchains

This document provides analysis of software supply chain attack
threats for the TF-A project

Change-Id: I03545d65a38dc372f3868a16c725b7378640a771
Signed-off-by: Lauren Wehrmeister <lauren.wehrmeister@arm.com>
12 files changed