PCI: apple: Move over to standalone probing

Now that we have the required infrastructure, split the Apple PCIe
setup into two categories:

- stuff that has to do with PCI setup stays in the .init() callback

- stuff that is just driver gunk (such as MSI setup) goes into a
  probe routine, which will eventually call into the host-common
  code

The result is a far more logical setup process.

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/20250401091713.2765724-6-maz@kernel.org