dt-bindings: pci: layerscape-pci: removed compatible string "snps,dw-pcie"

Removed the compatible string "snps,dw-pcie", it is for the reference
platform driver for PCI RC IP Protoyping Kits based on the ARC SDP,
so it is not suitable for all platform with designware PCIe controller,
and platform vendors have themselves' drivers.

The compatible string "snsp,dw-pcie" was added by mistake and it's not
matched that time, but it is matched because PCIe drivers has been
collected recently.

Signed-off-by: Hou Zhiqiang <Zhiqiang.Hou@nxp.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>