PCI: iproc: Add arm64 support

Add arm64 support to the iProc PCIe driver.

Note that on arm32, bus->sysdata points to the arm32-specific pci_sys_data
struct, and pci_sys_data.private_data contains the iproc_pcie pointer.
For arm64, there's nothing corresponding to pci_sys_data, so we keep the
iproc_pcie pointer directly in bus->sysdata.

In addition, arm64 does IRQ mapping in pcibios_add_device(), so it doesn't
need pci_fixup_irqs() as arm32 does.

Signed-off-by: Ray Jui <rjui@broadcom.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Scott Branden <sbranden@broadcom.com>