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/design/firmware-design.rst b/docs/design/firmware-design.rst
index ae6dd46..a357d58 100644
--- a/docs/design/firmware-design.rst
+++ b/docs/design/firmware-design.rst
@@ -957,6 +957,8 @@
 
 |Image 1|
 
+.. _handling-an-smc:
+
 Handling an SMC
 ~~~~~~~~~~~~~~~
 
@@ -1300,6 +1302,8 @@
 already been performed and act as appropriate. Possible courses of actions are,
 e.g. skip the action the second time, or undo/redo it.
 
+.. _configuring-secure-interrupts:
+
 Configuring secure interrupts
 -----------------------------