PCI: xgene-msi: Probe as a standard platform driver

Now that we have made the dependency between the PCI driver and
the MSI driver explicit, there is no need to use subsys_initcall()
as a probing hook, and we can rely on builtin_platform_driver()
instead.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lpieralisi@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://lore.kernel.org/r/20250708173404.1278635-12-maz@kernel.org

Author: Marc Zyngier

drivers/pci/controller/pci-xgene-msi.c

@@ -429,9 +429,4 @@ static struct platform_driver xgene_msi_driver = {
 	.probe = xgene_msi_probe,
 	.remove = xgene_msi_remove,
 };
-
-static int __init xgene_pcie_msi_init(void)
-{
-	return platform_driver_register(&xgene_msi_driver);
-}
-subsys_initcall(xgene_pcie_msi_init);
+builtin_platform_driver(xgene_msi_driver);