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>
diff --git a/drivers/pci/Kconfig b/drivers/pci/Kconfig
index ba41787..ab5a5e7 100644
--- a/drivers/pci/Kconfig
+++ b/drivers/pci/Kconfig
@@ -258,6 +258,10 @@
 	  Say Y here if you want to enable PCIe controller support on
 	  Armada XP/38x SoCs.
 
+config PCIE_DW_COMMON
+	bool
+	select DM_PCI
+
 config PCI_KEYSTONE
 	bool "TI Keystone PCIe controller"
 	depends on DM_PCI