PCI: rzg3s-host: Prepare System Controller handling for multiple controllers

Prepare the driver to handle multiple PCIe controllers with distinct
System Controller (SYSC) register sets, as required by RZ/V2H(P). The
current design stores a single sysc_info structure per SoC, which is
insufficient for multi-controller configurations.

Introduce controller identifiers and extend struct rzg3s_pcie_soc_data
to hold a sysc_info array indexed per PCIe controller. Add a
controller_id field to struct rzg3s_pcie_host and select the appropriate
System Controller information during probe based on the hardware
instance.

Keep existing single-controller SoCs functionally unchanged while
preparing the driver for RZ/V2H(P) multi-controller support.

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Tested-by: Claudiu Beznea <claudiu.beznea.uj@bp.renesas.com>
Reviewed-by: Claudiu Beznea <claudiu.beznea.uj@bp.renesas.com>
Link: https://patch.msgid.link/20260629220932.861445-4-prabhakar.mahadev-lad.rj@bp.renesas.com