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 <philmd@linaro.org>
Tested-by: Hsin-Yi Wang <hsinyi@chromium.org>
Tested-by: Mark-PK Tsai <mark-pk.tsai@mediatek.com>
Signed-off-by: Johan Hovold <johan+linaro@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230213104302.17307-15-johan+linaro@kernel.org

drivers/irqchip/irq-alpine-msi.c

@@ -204,7 +204,7 @@ static int alpine_msix_init_domains(struct alpine_msix_data *priv,
 		return -ENXIO;
 	}
 
-	middle_domain = irq_domain_add_tree(NULL,
+	middle_domain = irq_domain_add_hierarchy(gic_domain, 0, 0, NULL,
 						 &alpine_msix_middle_domain_ops,
 						 priv);
 	if (!middle_domain) {
@@ -212,8 +212,6 @@ static int alpine_msix_init_domains(struct alpine_msix_data *priv,
 		return -ENOMEM;
 	}
 
-	middle_domain->parent = gic_domain;
-
 	msi_domain = pci_msi_create_irq_domain(of_node_to_fwnode(node),
 					       &alpine_msix_domain_info,
 					       middle_domain);