hw/slw: use dt_new_check()

On P9 systems the /ibm,opal/power-mgt/ node may be created by the HDAT
parser before reaching add_cpu_idle_state_properties(). This is fine,
but it needs to be handled.

Signed-off-by: Oliver O'Halloran <oohall@gmail.com>
Reviewed-by: Vasant Hegde <hegdevasant@linux.vnet.ibm.com>
Signed-off-by: Stewart Smith <stewart@linux.vnet.ibm.com>

hw/slw.c

@@ -648,11 +648,10 @@ void add_cpu_idle_state_properties(void)
 	u32 *residency_ns_buf;
 	u32 *flags_buf;
 
-
 	prlog(PR_DEBUG, "CPU idle state device tree init\n");
 
-	/* Create /ibm,opal/power-mgt */
-	power_mgt = dt_new(opal_node, "power-mgt");
+	/* Create /ibm,opal/power-mgt if it doesn't exist already */
+	power_mgt = dt_new_check(opal_node, "power-mgt");
 	if (!power_mgt) {
 		/**
 		 * @fwts-label CreateDTPowerMgtNodeFail