PCI: Add per-device MSI domain hook

So far, we have considered that the MSI domain for a device was
either set via the architecture-dependent pcibios implementation
or inherited from the host bridge.

As we're about to break that assumption, add pci_dev_msi_domain
which is the equivalent of pci_host_bridge_msi_domain, but for
a single device.

Other than moving things around a bit, this patch on its own
has no effect.

Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>