Documentation: Move L1TF to separate directory

Move L!TF to a separate directory so the MDS stuff can be added at the
side. Otherwise the all hardware vulnerabilites have their own top level
entry. Should have done that right away.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Jon Masters <jcm@redhat.com>