Add documentation for the SRBDS vulnerability and its mitigation.

 [ bp: Massage.
   jpoimboe: sysfs table strings. ]
Signed-off-by: Mark Gross <mgross@linux.intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Reviewed-by: Josh Poimboeuf <jpoimboe@redhat.com>

CVE-2020-0543
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Marcelo Cerri <marcelo.cerri@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>