KVM: s390: Add diag "watchdog functions" to trace event decoding

DIAG 0x288 may occur now. Let's add its code to the diag table in
sie.h.

Signed-off-by: Alexander Yarygin <yarygin@linux.vnet.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>