threat_model: Change the format and contents
Change the format of the threat model to markdown for consistency with the firmware documentation. Also, the threat model and security requirements are updated to reflect more tangible attacks.
Signed-off-by: Tarek El-Sherbiny tarek.el-sherbiny@arm.com Change-Id: I01bc8f9b10d1085e0c236687ed26995ee26f6d26