Documentation: Add "S390" to the swiotlb kernel parameter

The "swiotlb" kernel parameter is used on s390 for protected virt since
commit 64e1f0c5 ("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

Documentation/admin-guide/kernel-parameters.txt

@@ -6526,7 +6526,7 @@
 			This parameter controls use of the Protected
 			Execution Facility on pSeries.
 
-	swiotlb=	[ARM,IA-64,PPC,MIPS,X86,EARLY]
+	swiotlb=	[ARM,IA-64,PPC,MIPS,X86,S390,EARLY]
 			Format: { <int> [,<int>] | force | noforce }
 			<int> -- Number of I/O TLB slabs
 			<int> -- Second integer after comma. Number of swiotlb