PCI: dwc: Add dw_pcie_ep_{read,write}_dbi[2] helpers

PCI: dwc: Add dw_pcie_ep_{read,write}_dbi[2] helpers

The current code calculated some dbi[2] registers' offset by calling
dw_pcie_ep_get_dbi[2]_offset() in each function. To improve the code
readability, add dw_pcie_ep_{read,write}_dbi[2} and some data-width
related helpers.

Link: https://lore.kernel.org/linux-pci/20231220053829.1921187-5-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>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>