Documentation: Add section about CPU vulnerabilities

Documentation: Add section about CPU vulnerabilities

Add documentation for the L1TF vulnerability and the mitigation mechanisms:

  - Explain the problem and risks
  - Document the mitigation mechanisms
  - Document the command line controls
  - Document the sysfs files

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Josh Poimboeuf <jpoimboe@redhat.com>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lkml.kernel.org/r/20180713142323.287429944@linutronix.de
Orabug: 28220674
CVE: CVE-2018-3620

(cherry picked from commit 3ec8ce5d866ec6a08a9cfab82b62acf4a830b35f)

Signed-off-by: Mihai Carabas <mihai.carabas@oracle.com>
Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Conflicts:
	Documentation/admin-guide/index.rst
	Documentation/admin-guide/l1tf.rst
Contextual: The admin-guide is introduced later by 9d85025b "docs-rst: create
an user's manual book" so l1tf is added in Documentation folder.