xen-platform: use builtin_pci_driver

Use builtin_pci_driver() helper to simplify the code.
Signed-off-by: Geliang Tang <geliangtang@gmail.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>

drivers/xen/platform-pci.c

@@ -125,8 +125,4 @@ static struct pci_driver platform_driver = {
 	.id_table =       platform_pci_tbl,
 };
 
-static int __init platform_pci_init(void)
-{
-	return pci_register_driver(&platform_driver);
-}
-device_initcall(platform_pci_init);
+builtin_pci_driver(platform_driver);