dt-bindings: pci: layerscape-pci: Add a optional property big-endian

This property is to indicate the endianness when accessing the
PEX_LUT and PF register block, so if these registers are
implemented in big-endian, specify this property.

Link: https://lore.kernel.org/r/20220311234938.8706-2-leoyang.li@nxp.com
Signed-off-by: Hou Zhiqiang <Zhiqiang.Hou@nxp.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/pci/layerscape-pci.txt

@@ -40,6 +40,10 @@ Required properties:
   of the data transferred from/to the IP block. This can avoid the software
   cache flush/invalid actions, and improve the performance significantly.
 
+Optional properties:
+- big-endian: If the PEX_LUT and PF register block is in big-endian, specify
+  this property.
+
 Example:
 
 	pcie@3400000 {