Add a msi_controller setup_irqs() method so MSI chip providers can
implement their own multivector MSI setup.

[bhelgaas: changelog]
Signed-off-by: NLucas Stach <l.stach@pengutronix.de>
Signed-off-by: NBjorn Helgaas <bhelgaas@google.com>
Reviewed-by: NPratyush Anand <pratyush.anand@gmail.com>