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: [e-mail]
Reviewed-by: Rob Herring <[e-mail]>
Signed-off-by: Linus Walleij <[e-mail]>
Signed-off-by: Miquel Raynal <[e-mail]>

Documentation/devicetree/bindings/mtd/intel,ixp4xx-flash.txt

···
+Flash device on Intel IXP4xx SoC
+
+This flash is regular CFI compatible (Intel or AMD extended) flash chips with
+specific big-endian or mixed-endian memory access pattern.
+
+Required properties:
+- compatible : must be "intel,ixp4xx-flash", "cfi-flash";
+- reg : memory address for the flash chip
+- bank-width : width in bytes of flash interface, should be <2>
+
+For the rest of the properties, see mtd-physmap.txt.
+
+The device tree may optionally contain sub-nodes describing partitions of the
+address space. See partition.txt for more detail.
+
+Example:
+
+flash@50000000 {
+	compatible = "intel,ixp4xx-flash", "cfi-flash";
+	reg = <0x50000000 0x01000000>;
+	bank-width = <2>;
+};