PCI: layerscape: Add support for link-down notification

Add support to pass link-down notification to the endpoint function
driver so that it can process the LINK_DOWN event.

Link: https://lore.kernel.org/r/20230720135834.1977616-1-Frank.Li@nxp.com
Signed-off-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Lorenzo Pieralisi <lpieralisi@kernel.org>
Acked-by: Manivannan Sadhasivam <mani@kernel.org>