Fix broken links to various sections across docs

These broken links were found with the help of this command:
$> sphinx-build -M linkcheck . build

A sample broken link is reported as follows:
(line   80) -local-   firmware-design.rst#secure-el1-payloads-and-dispatchers

Change-Id: I5dcefdd4b8040908658115647e957f6c2c5da7c2
Signed-off-by: Madhukar Pappireddy <madhukar.pappireddy@arm.com>
diff --git a/docs/components/sdei.rst b/docs/components/sdei.rst
index c5275a0..60259c8 100644
--- a/docs/components/sdei.rst
+++ b/docs/components/sdei.rst
@@ -205,6 +205,8 @@
 
 See the function ``sdei_client_el()`` in ``sdei_private.h``.
 
+.. _explicit-dispatch-of-events:
+
 Explicit dispatch of events
 ---------------------------