Docs: threat_models: Style and refs updates

- Update the Data Flow Diagram (r/NPSE/NSPE)
- Fix TF-M version for v1.4 in version control table
- Minor cosmetics improvements

Signed-off-by: Nicola Mazzucato <nicola.mazzucato@arm.com>
Change-Id: I807e8c4dcb20c191d2f15d215d511602f77b4660
5 files changed