of: make PowerMac cache node search conditional on CONFIG_PPC_PMAC

Cache nodes under the cpu node(s) is PowerMac specific according to the
comment above, so make the code enforce that.
Signed-off-by: Rob Herring <robh@kernel.org>

drivers/of/base.c

@@ -2010,7 +2010,7 @@ struct device_node *of_find_next_cache_node(const struct device_node *np)
 	/* OF on pmac has nodes instead of properties named "l2-cache"
 	 * beneath CPU nodes.
 	 */
-	if (!strcmp(np->type, "cpu"))
+	if (IS_ENABLED(CONFIG_PPC_PMAC) && !strcmp(np->type, "cpu"))
 		for_each_child_of_node(np, child)
 			if (!strcmp(child->type, "cache"))
 				return child;