docs: Refactor generic threat model

* Replace tables by list-tables for easier maintenance
* Add labels for enabling references from other service specific threat
  models
* Clean up generic data flow graph

Signed-off-by: Imre Kis <imre.kis@arm.com>
Change-Id: I8eed776c5b2c9e8cfd6b46b1c94f8b29b5cac319
3 files changed