docs: add SIMD support section

Add hafnium design doc section related to SIMD state preservation by the
SPMC.

Signed-off-by: Olivier Deprez <olivier.deprez@arm.com>
Change-Id: I32e358926eb3e062a4e94c0455879da029f68ce6
diff --git a/docs/glossary.rst b/docs/glossary.rst
index 265e4f3..d4e7280 100644
--- a/docs/glossary.rst
+++ b/docs/glossary.rst
@@ -139,6 +139,9 @@
    SEA
       Synchronous External Abort
 
+   SIMD
+      Single Instruction Multiple Data
+
    SiP
       Silicon Provider
 
@@ -169,6 +172,9 @@
    SVE
       Scalable Vector Extension
 
+   SME
+      Scalable Matrix Extension
+
    SWd
       Secure World