pci: add common Designware PCIe functions

pci: add common Designware PCIe functions

With the introduction of pcie_dw_rockchip, and need to support the DW PCIe in the
Amlogic AXG & G12 SoCs, most of the DW PCIe helpers would be duplicated.

This introduce a "common" DW PCIe helpers file with common code merged from the
dw_ti and dw_rockchip drivers and adapted to fit with the upcoming dw_meson.

The following changes will switch the dw_ti and dw_rockchip to use these helpers.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Tested-by: Green Wan <green.wan@sifive.com>
[bmeng: remove the blank line at EOF of drivers/pci/pcie_dw_common.c]
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>