docs(threat-model): provide PSR specification reference

Added an assumption in generic threat model that platform's hardware
conforms the Platform Security Requirements specification.

Change-Id: I753287feec1cd459edfd3d1c103e0e701827cc05
Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>
diff --git a/docs/glossary.rst b/docs/glossary.rst
index 58b7d99..679de2b 100644
--- a/docs/glossary.rst
+++ b/docs/glossary.rst
@@ -142,6 +142,9 @@
    PSA
       Platform Security Architecture
 
+   PSR
+     Platform Security Requirements
+
    PSCI
       Power State Coordination Interface