stm class: Add a reference to the SyS-T document

Commit 4cb3653d ("stm class: Document the MIPI SyS-T protocol usage")
added a document describing the SyS-T protocol usage, but forgot to add
it to the directory index. Fix that.

Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Fixes: 4cb3653d ("stm class: Document the MIPI SyS-T protocol usage")
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>