pci: layerscape: move kernel DT fixup to a separate file

To make the layerscape pcie driver clear, move the kernel DT fixup
code from pcie_layerscape.c to pcie_layerscape_fixup.c.

Signed-off-by: Hou Zhiqiang <Zhiqiang.Hou@nxp.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: York Sun <york.sun@nxp.com>