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