PCI: dwc: Expose dw_pcie_write_dbi2() to module

PCI: dwc: Expose dw_pcie_write_dbi2() to module

Since no PCIe controller drivers call this, this change is not required
for now. But, Renesas R-Car Gen4 PCIe controller driver will call this
and if the controller driver is built as a kernel module, the following
build error happens:

  ERROR: modpost: "dw_pcie_write_dbi2" [drivers/pci/controller/dwc/pcie-rcar-gen4-host-drv.ko] undefined!

So, expose dw_pcie_write_dbi2() for it.

Link: https://lore.kernel.org/linux-pci/20230926122431.3974714-8-yoshihiro.shimoda.uh@renesas.com
Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Reviewed-by: Serge Semin <fancer.lancer@gmail.com>