PCI: ecam: Allow cfg->priv to be pre-populated from the root port device

In order to decouple ecam config space creation from probing via
pci_host_common_probe(), allow the private pointer to be populated
via the device drvdata pointer.

Crucially, this is set before calling ops->init(), allowing that
particular callback to have access to probe data.

This should have no impact on existing code which ignores the
current value of cfg->priv.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Tested-by: Janne Grunau <j@jannau.net>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Acked-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Link: https://patch.msgid.link/20250401091713.2765724-5-maz@kernel.org