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-mm.rst b/docs/components/secure-partition-manager-mm.rst
index 87fc91d..d532901 100644
--- a/docs/components/secure-partition-manager-mm.rst
+++ b/docs/components/secure-partition-manager-mm.rst
@@ -6,11 +6,9 @@
 
 Two implementations of a Secure Partition Manager co-exist in the TF-A codebase:
 
--  SPM based on the PSA FF-A specification (`Secure Partition Manager`__).
+-  SPM based on the PSA FF-A specification (:ref:`Secure Partition Manager`).
 -  SPM based on the MM interface.
 
-.. __: secure-partition-manager.html
-
 Both implementations differ in their architectures and only one can be selected
 at build time.