mtd: add DT bindings for the Intel IXP4xx Flash

This adds device tree bindings for the Intel IXP4xx
flash controller, a simple physmap which however need a
specific big-endian or mixed-endian access pattern to the
memory.

Cc: devicetree@vger.kernel.org
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>