ARM/PCI: Save MSI controller in pci_sys_data

Currently ARM associates an MSI controller with a PCI bus by defining
pcibios_add_bus() and using it to call a struct hw_pci.add_bus() method.
That method sets the struct pci_bus "msi" member.  That's unwieldy and
unnecessarily couples MSI with the PCI enumeration code.

On ARM, all devices under the same PCI host bridge share an MSI controller,
so add an msi_controller pointer to the struct pci_sys_data and implement
pcibios_msi_controller() to retrieve it.

This is a step toward moving the msi_controller pointer into the generic
struct pci_host_bridge.

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