powercap: dtpm: Simplify with of_machine_get_match_data()

Replace open-coded getting root OF node, matching against it and getting
the match data with new of_machine_get_match_data() helper.

Signed-off-by: Krzysztof Kozlowski <[e-mail]>
Reviewed-by: Lukasz Luba <[e-mail]>
Reviewed-by: AngeloGioacchino Del Regno <[e-mail]>
Tested-by: AngeloGioacchino Del Regno <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Rob Herring (Arm) <[e-mail]>

drivers/powercap/dtpm.c

···
  */
 int dtpm_create_hierarchy(struct of_device_id *dtpm_match_table)
 {
-	const struct of_device_id *match;
 	const struct dtpm_node *hierarchy;
-	struct device_node *np;
 	int i, ret;
 
 	mutex_lock(&dtpm_lock);
···
 		goto out_pct;
 	}
 
-	ret = -ENODEV;
-	np = of_find_node_by_path("/");
-	if (!np)
-		goto out_err;
-
-	match = of_match_node(dtpm_match_table, np);
-
-	of_node_put(np);
-
-	if (!match)
-		goto out_err;
-
-	hierarchy = match->data;
+	hierarchy = of_machine_get_match_data(dtpm_match_table);
 	if (!hierarchy) {
 		ret = -EFAULT;
 		goto out_err;