feat(plat/fvp_r):  Threat Model for TF-A v8-R64 Support

Threat model for the current, BL1-only R-class support.

Signed-off-by: Gary Morrison <gary.morrison@arm.com>
Change-Id: I8479d5cb30f3cf3919281cc8dc1f21cada9511e0
diff --git a/docs/threat_model/index.rst b/docs/threat_model/index.rst
index b5ede69..335937e 100644
--- a/docs/threat_model/index.rst
+++ b/docs/threat_model/index.rst
@@ -15,6 +15,7 @@
 
    threat_model
    threat_model_spm
+   threat_model_fvp_r
 
 --------------