mtd: hisilicon: add device tree binding documentation

This patch adds the related dts binding document for Hisilicon 504 NAND
controller.
Signed-off-by: Zhou Wang <wangzhou1@hisilicon.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>

Documentation/devicetree/bindings/mtd/hisi504-nand.txt

+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>;
+		};
+
+		...
+
+	};