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: Prabhakar Kushwaha <prabhakar.kushwaha@nxp.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>