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/secure-partition-manager.rst b/docs/components/secure-partition-manager.rst
index 2169f30..e09da53 100644
--- a/docs/components/secure-partition-manager.rst
+++ b/docs/components/secure-partition-manager.rst
@@ -833,9 +833,7 @@
 
 .. _[2]:
 
-[2] `Secure Partition Manager using MM interface`__
-
-.. __: secure-partition-manager-mm.html
+[2] :ref:`Secure Partition Manager using MM interface<Secure Partition Manager (MM)>`
 
 .. _[3]: