Documentation: Add "S390" to the swiotlb kernel parameter

Documentation: Add "S390" to the swiotlb kernel parameter

The "swiotlb" kernel parameter is used on s390 for protected virt since
commit 64e1f0c531d1 ("s390/mm: force swiotlb for protected virtualization")
and thus should be marked in kernel-parameters.txt accordingly.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Acked-by: Christian Borntraeger <borntraeger@linux.ibm.com>
Acked-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240614081438.553160-1-thuth@redhat.com