PCI: rcar-gen4: Add struct rcar_gen4_pcie_drvdata

PCI: rcar-gen4: Add struct rcar_gen4_pcie_drvdata

In order to support future SoCs such as r8a779g0 (R-Car V4H) and
r8a779h0 (R-Car V4M) that require different initialization settings,
introduce SoC specific driver data with the initial member being the
device mode.

No functional change.

[kwilczynski: commit log]
Link: https://lore.kernel.org/linux-pci/20240611125057.1232873-3-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: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>