PCI: dwc: designware: Move _unroll configurations to a separate function

No functional change. Rename dw_pcie_writel_unroll/dw_pcie_readl_unroll to
dw_pcie_writel_ob_unroll/dw_pcie_readl_ob_unroll respectively as these
functions are used to perform only outbound configurations. Also move
these _unroll configurations to a separate function.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>