PCI: rzg3s-host: Make SYSC register offsets SoC-specific

PCI: rzg3s-host: Make SYSC register offsets SoC-specific

In preparation for adding RZ/G3E support, move the RST_RSM_B register
offset and mask into a SoC-specific data structure. Compared with RZ/G3S,
the RZ/G3E SYSC controls different functionalities for the PCIe controller.

Make SYSC operations conditional on the presence of register offset
information, allowing the driver to handle SoCs that don't use the
RST_RSM_B signal.

Signed-off-by: John Madieu <john.madieu.xa@bp.renesas.com>
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Tested-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com> # RZ/V2N EVK
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/20260306143423.19562-8-john.madieu.xa@bp.renesas.com