docs(amu): add AMU documentation

This change adds some documentation on the AMU and its purpose. This is
expanded on in later patches.

Change-Id: If2834676790938d8da5ea2ceba37b674f6cc0f01
Signed-off-by: Chris Kay <chris.kay@arm.com>
diff --git a/docs/glossary.rst b/docs/glossary.rst
index f4912f5..a062319 100644
--- a/docs/glossary.rst
+++ b/docs/glossary.rst
@@ -15,6 +15,10 @@
    AArch64
       64-bit execution state of the ARMv8 ISA
 
+   AMU
+      Activity Monitor Unit, a hardware monitoring unit introduced by FEAT_AMUv1
+      that exposes CPU core runtime metrics as a set of counter registers.
+
    API
       Application Programming Interface