Previously __dw_pcie_prog_outbound_atu() duplicated a lot of code between
the iatu_unroll_enabled version and the PCIE_ATU_VIEWPORT version:

  __dw_pcie_prog_outbound_atu
    if (iatu_unroll_enabled)
      dw_pcie_prog_outbound_atu_unroll
        dw_pcie_writel_ob_unroll(PCIE_ATU_UNR_LOWER_BASE, ...)
        dw_pcie_writel_ob_unroll(PCIE_ATU_UNR_UPPER_BASE, ...)
        ...
      return
    dw_pcie_writel_dbi(PCIE_ATU_VIEWPORT, ...)
    dw_pcie_writel_dbi(PCIE_ATU_LOWER_BASE, ...)
    dw_pcie_writel_dbi(PCIE_ATU_UPPER_BASE, ...)
    ...

Unify those by pushing the unroll address computation and viewport
selection down into dw_pcie_writel_atu() so we can use the same
dw_pcie_writel_atu_ob() accessor for both paths:

  __dw_pcie_prog_outbound_atu
    dw_pcie_writel_atu_ob(PCIE_ATU_LOWER_BASE, ...)
      dw_pcie_writel_atu
        dw_pcie_select_atu                      # new
          if (iatu_unroll_enabled)
            return pci->atu_base + PCIE_ATU_UNROLL_BASE(...)
          dw_pcie_writel_dbi(PCIE_ATU_VIEWPORT, ...)
          return pci->atu_base
        dw_pcie_write(base + reg)
      dw_pcie_writel_atu_ob(PCIE_ATU_UPPER_BASE, ...)
      ...

In the non-unroll case, this does involve more MMIO writes to
PCIE_ATU_VIEWPORT, but it's mainly in initialization paths and the code
simplification is significant.

[bhelgaas: commit log, simplify dw_pcie_select_atu()]
Link: https://lore.kernel.org/r/20220624143947.8991-12-Sergey.Semin@baikalelectronics.ruSigned-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>