PCI: layerscape: Add the endpoint linkup notifier support

Layerscape has PME interrupt, which can be used as linkup notifier.  Set
CFG_READY bit of PEX_PF0_CONFIG to enable accesses from root complex when
linkup detected.

Link: https://lore.kernel.org/r/20230515151049.2797105-1-Frank.Li@nxp.com
Signed-off-by: Xiaowei Bao <xiaowei.bao@nxp.com>
Signed-off-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Lorenzo Pieralisi <lpieralisi@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>