docs(changelog): add subsection common

Introduce subsection common under Arm.

Change-Id: I24177b5ab1636af56aab3c55b21f2ca014546b9c
Signed-off-by: Govindraj Raja <govindraj.raja@arm.com>
diff --git a/changelog.yaml b/changelog.yaml
index f2403d0..9d1c3a9 100644
--- a/changelog.yaml
+++ b/changelog.yaml
@@ -194,6 +194,9 @@
           - plat/arm
 
         subsections:
+          - title: Common
+            scope: common
+
           - title: A5DS
             scope: a5ds