Documentation: Add documentation for Processor MMIO Stale Data

Add the admin guide for Processor MMIO stale data vulnerabilities.
Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>

Documentation/admin-guide/hw-vuln/index.rst

@@ -17,3 +17,4 @@ are configurable at compile, boot or run time.
    special-register-buffer-data-sampling.rst
    core-scheduling.rst
    l1d_flush.rst
+   processor_mmio_stale_data.rst