KVM: s390: add documentation of KVM_S390_VM_TOD

Let's properly document KVM_S390_VM_TOD and its attributes.

Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>