Documentation: fsl-quadspi: Add optional properties

Add optional properties for QSPI:
big-endian
if the register is big endian on this platform.

Signed-off-by: Yuan Yao <yao.yuan@nxp.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Han xu <han.xu@nxp.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>

Documentation/devicetree/bindings/mtd/fsl-quadspi.txt

@@ -21,6 +21,7 @@ Optional properties:
 			      But if there are two NOR flashes connected to the
 			      bus, you should enable this property.
 			      (Please check the board's schematic.)
+  - big-endian : That means the IP register is big endian
 
 Example: