docs(porting): refer the reader back to the threat model

docs(porting): refer the reader back to the threat model

When porting TF-A to a new platform, it is essential to read the
threat model documents in conjunction with the porting guide to
understand the security responsibilities of each platform interface
to implement.

Add a note to highlight this in the porting guide.

Change-Id: Icd1e41ae4b15032b72531690dd82a9ef95ca0db5
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>