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