PCI/MSI: Allow an msi_controller to be associated to an irq domain

With the new stacked irq domains, it becomes pretty tempting to
allocate an MSI domain per PCI bus, which would remove the requirement
of either relying on arch-specific code, or a default PCI MSI domain.

By allowing the msi_controller structure to carry a pointer to an
irq_domain, we can easily use this in pci_msi_setup_msi_irqs.  The
existing code can still be used as a fallback if the MSI driver does
not populate the domain field.

Tested on arm64 with the GICv3 ITS driver.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Cc: Yingjoe Chen <yingjoe.chen@mediatek.com>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: Jiang Liu <jiang.liu@linux.intel.com>
Link: http://lkml.kernel.org/r/1416048553-29289-2-git-send-email-marc.zyngier@arm.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>