ARM/PCI: Remove msi_controller from struct pci_sys_data

ARM now uses pci_bus->msi to store the msi_controller pointer, so we don't
need to save it in struct pci_sys_data, and we don't need to implement
pcibios_msi_controller() to get it out of pci_sys_data.

Remove msi_controller from struct pci_sys_data and
pcibios_msi_controller().

[bhelgaas: changelog, split into separate patch]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Jingoo Han <jingoohan1@gmail.com>