PCI: rzg3s-host: Add SoC-specific configuration and initialization callbacks

PCI: rzg3s-host: Add SoC-specific configuration and initialization callbacks

Add optional cfg_pre_init, cfg_post_init, and cfg_deinit callbacks
to handle SoC-specific configuration methods. While RZ/G3S uses the Linux
reset framework with dedicated reset lines, other SoC variants like RZ/G3E
control configuration resets through PCIe AXI registers.

As Linux reset bulk API gracefully handles optional NULL reset lines
(num_cfg_resets = 0 for RZ/G3E), the driver continues to use the standard
reset framework when reset lines are available, while custom callbacks
are only invoked when provided.

This provides a balanced pattern where:
- RZ/G3S: Uses callbacks that fall back to the reset framework
- RZ/G3E: Sets num_cfg_resets=0, provides
  cfg_pre_init/cfg_post_init/cfg_deinit

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/20260306143423.19562-10-john.madieu.xa@bp.renesas.com