PCI/MSI: Add weak pcibios_msi_controller()

Add pcibios_msi_controller() to get the msi_controller associated with a
PCI device.  This is to allow arches to store the msi_controller in the
arch-specific PCI sysdata.

[bhelgaas: changelog, take pci_dev instead of pci_bus]
Suggested-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Yijing Wang <wangyijing@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>