docs(changelog): changelog for v2.13 release

Generated this change-log using below command:
npm run release -- --skip.commit --skip.tag --release-as 2.13.0

Change-Id: Ibb0623f04c641b65a03deaffd50a5a8b65637419
Signed-off-by: Govindraj Raja <govindraj.raja@arm.com>
diff --git a/Makefile b/Makefile
index 150aa30..530e6e0 100644
--- a/Makefile
+++ b/Makefile
@@ -8,7 +8,7 @@
 # Trusted Firmware Version
 #
 VERSION_MAJOR			:= 2
-VERSION_MINOR			:= 12
+VERSION_MINOR			:= 13
 # VERSION_PATCH is only used for LTS releases
 VERSION_PATCH			:= 0
 VERSION				:= ${VERSION_MAJOR}.${VERSION_MINOR}.${VERSION_PATCH}