docs: Update docs with firmware encryption feature

Update documentation with optional firmware encryption feature.

Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
Change-Id: I26691b18e1ee52a73090954260f26f2865c4e05a
diff --git a/docs/change-log-upcoming.rst b/docs/change-log-upcoming.rst
index c4e8bb0..15f39de 100644
--- a/docs/change-log-upcoming.rst
+++ b/docs/change-log-upcoming.rst
@@ -46,6 +46,7 @@
 
 - Security
    - Example: "UBSAN support and handlers"
+   - Add support for optional firmware encryption feature (experimental).
 
 - Tools
    - Example: "fiptool: Add support to build fiptool on Windows."