dt-bindings: mtd-physmap: Add endianness supports

Provide a way to specify the endianness to use when accessing a
memory-mapped flash.
Signed-off-by: NPrabhakar Kushwaha <prabhakar.kushwaha@nxp.com>
Reviewed-by: NRob Herring <robh@kernel.org>
Signed-off-by: NBoris Brezillon <boris.brezillon@bootlin.com>

Documentation/devicetree/bindings/mtd/mtd-physmap.txt

@@ -41,6 +41,13 @@ additional (optional) property is defined:
 
  - erase-size : The chip's physical erase block size in bytes.
 
+ The device tree may optionally contain endianness property.
+ little-endian or big-endian : It Represents the endianness that should be used
+                               by the controller to  properly read/write data
+			       from/to the flash. If this property is missing,
+			       the endianness is chosen by the system
+			       (potentially based on extra configuration options).
+
 The device tree may optionally contain sub-nodes describing partitions of the
 address space. See partition.txt for more detail.