tpm: move documentation under Documentation/security

In order too make Documentation root directory cleaner move the tpm
directory under Documentation/security.
Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/index.rst

@@ -19,7 +19,7 @@ Contents:
    media/dvb-drivers/index
    media/v4l-drivers/index
    gpu/index
-   tpm/index
+   security/index
 
 Indices and tables
 ==================

Documentation/security/index.rst

+======================
+Security documentation
+======================
+
+.. toctree::
+
+   tpm/index