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>