The sparse checker complains about converting pointers between address
spaces.  The pci_config_window.priv pointer is a generic void *, but
hisi_pcie_map_bus() needs a void __iomem *.

This isn't a problem in other drivers because they store the __iomem
pointer in a driver struct.  Add a trivial struct hisi_pcie to avoid the
warning.

The sparse warning looks like this:

  $ make C=2 drivers/pci/controller/
  drivers/pci/controller/dwc/pcie-hisi.c:61:37: warning: incorrect type in initializer (different address spaces)
  drivers/pci/controller/dwc/pcie-hisi.c:61:37:    expected void [noderef] __iomem *reg_base
  drivers/pci/controller/dwc/pcie-hisi.c:61:37:    got void *priv

Link: https://lore.kernel.org/r/20211223213749.1314142-2-helgaas@kernel.orgSigned-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Zhou Wang <wangzhou1@hisilicon.com>