PCI: designware: Make dw_pcie_rd_own_conf(), etc., static

The following variables and functions are used only in pcie-designware.c,
so make them static:

  global_io_offset
  dw_pcie_rd_own_conf()
  dw_pcie_wr_own_conf()
  dw_pcie_setup()
  dw_pcie_scan_bus()
  dw_pcie_map_irq()

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Jingoo Han <jg1.han@samsung.com>