PCI: layerscape-ep: Use the generic dw_pcie_ep_linkdown() API to handle Link Down event

Now that dw_pcie_ep_linkdown() is available, use it. This also handles the
reinitialization of DWC non-sticky registers in addition to sending the
notification to EPF drivers.

Closes: https://lore.kernel.org/linux-pci/20240528195539.GA458945@bhelgaas
Link: https://lore.kernel.org/linux-pci/20240606-pci-deinit-v1-5-4395534520dc@linaro.org


Reported-by: Bjorn Helgaas <helgaas@kernel.org>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Reviewed-by: Niklas Cassel <cassel@kernel.org>