mtd: hisilicon: add device tree binding documentation

Hisilicon Hip04 Soc NAND controller DT binding

Required properties:

- compatible:          Should be "hisilicon,504-nfc".

- reg:                 The first contains base physical address and size of

                       NAND controller's registers. The second contains base

                       physical address and size of NAND controller's buffer.

- interrupts:          Interrupt number for nfc.

- nand-bus-width:      See nand.txt.

- nand-ecc-mode:       Support none and hw ecc mode.

- #address-cells:      Partition address, should be set 1.

- #size-cells:         Partition size, should be set 1.

Optional properties:

- nand-ecc-strength:   Number of bits to correct per ECC step.

- nand-ecc-step-size:  Number of data bytes covered by a single ECC step.

The following ECC strength and step size are currently supported:

 - nand-ecc-strength = <16>, nand-ecc-step-size = <1024>

Flash chip may optionally contain additional sub-nodes describing partitions of

the address space. See partition.txt for more detail.

Example:

	nand: nand@4020000 {

		compatible = "hisilicon,504-nfc";

		reg = <0x4020000 0x10000>, <0x5000000 0x1000>;

		interrupts = <0 379 4>;

		nand-bus-width = <8>;

		nand-ecc-mode = "hw";

		nand-ecc-strength = <16>;

		nand-ecc-step-size = <1024>;

		#address-cells = <1>;

		#size-cells = <1>;

		partition@0 {

			label = "nand_text";

			reg = <0x00000000 0x00400000>;

		};

		...

	};