PCI: rzg3s-host: Add PCIe Gen3 (8.0 GT/s) link speed support

PCI: rzg3s-host: Add PCIe Gen3 (8.0 GT/s) link speed support

Extend the link speed configuration to support Gen3 (8.0 GT/s) in
addition to Gen2 (5.0 GT/s). This is required for RZ/G3E PCIe host
support, which is Gen3 capable.

Instead of relying on DT max-link-speed for configuration, read the
hardware capabilities from the PCI_EXP_LNKCAP register to determine the
maximum supported speed. The DT max-link-speed property is now only used
as an optional limit when explicitly specified, which aligns with PCIe
subsystem expectations.

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/20260306143423.19562-12-john.madieu.xa@bp.renesas.com