doc: Update description for AT speculative workaround

Documented the CPU specific build macros created for AT
speculative workaround.

Updated the description of 'ERRATA_SPECULATIVE_AT' errata
workaround option.

Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>
Change-Id: Ie46a80d4e8183c1d5c8b153f08742a04d41a2af2
diff --git a/docs/glossary.rst b/docs/glossary.rst
index e087079..08add3a 100644
--- a/docs/glossary.rst
+++ b/docs/glossary.rst
@@ -18,6 +18,9 @@
    API
       Application Programming Interface
 
+   AT
+      Address Translation
+
    BTI
       Branch Target Identification. An Armv8.5 extension providing additional
       control flow integrity around indirect branches and their targets.