PCI: apple: Add tracking of probed root ports

The apple driver relies on being able to directly find the matching root
port structure from the platform device that represents this port.

A previous hack stashed a pointer to the root port structure in the config
window private pointer, but that ended up relying on assumptions that break
other drivers.

Instead, bite the bullet and track the association as part of the driver
itself as a list of probed root ports.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://patch.msgid.link/20250625111806.4153773-3-maz@kernel.org