irqchip/alpine-msi: Use irq_domain_add_hierarchy()

Use the irq_domain_add_hierarchy() helper to create the hierarchical
domain, which both serves as documentation and avoids poking at
irqdomain internals.

Reviewed-by: Philippe Mathieu-Daudé <[e-mail]>
Tested-by: Hsin-Yi Wang <[e-mail]>
Tested-by: Mark-PK Tsai <[e-mail]>
Signed-off-by: Johan Hovold <[e-mail]>
Signed-off-by: Marc Zyngier <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

drivers/irqchip/irq-alpine-msi.c

···
 		return -ENXIO;
 	}
 
-	middle_domain = irq_domain_add_tree(NULL,
-					    &alpine_msix_middle_domain_ops,
-					    priv);
+	middle_domain = irq_domain_add_hierarchy(gic_domain, 0, 0, NULL,
+						 &alpine_msix_middle_domain_ops,
+						 priv);
 	if (!middle_domain) {
 		pr_err("Failed to create the MSIX middle domain\n");
 		return -ENOMEM;
 	}
-
-	middle_domain->parent = gic_domain;
 
 	msi_domain = pci_msi_create_irq_domain(of_node_to_fwnode(node),
 					       &alpine_msix_domain_info,