mtd: nand: add sunxi NFC dt bindings doc

Add the sunxi NAND Flash Controller dt bindings documentation.
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>

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

+Allwinner NAND Flash Controller (NFC)
+
+Required properties:
+- compatible : "allwinner,sun4i-a10-nand".
+- reg : shall contain registers location and length for data and reg.
+- interrupts : shall define the nand controller interrupt.
+- #address-cells: shall be set to 1. Encode the nand CS.
+- #size-cells : shall be set to 0.
+- clocks : shall reference nand controller clocks.
+- clock-names : nand controller internal clock names. Shall contain :
+    * "ahb" : AHB gating clock
+    * "mod" : nand controller clock
+
+Optional children nodes:
+Children nodes represent the available nand chips.
+
+Optional properties:
+- allwinner,rb : shall contain the native Ready/Busy ids.
+ or
+- rb-gpios : shall contain the gpios used as R/B pins.
+- nand-ecc-mode : one of the supported ECC modes ("hw", "hw_syndrome", "soft",
+  "soft_bch" or "none")
+
+see Documentation/devicetree/mtd/nand.txt for generic bindings.
+
+
+Examples:
+nfc: nand@01c03000 {
+	compatible = "allwinner,sun4i-a10-nand";
+	reg = <0x01c03000 0x1000>;
+	interrupts = <0 37 1>;
+	clocks = <&ahb_gates 13>, <&nand_clk>;
+	clock-names = "ahb", "mod";
+	#address-cells = <1>;
+	#size-cells = <0>;
+	pinctrl-names = "default";
+	pinctrl-0 = <&nand_pins_a &nand_cs0_pins_a &nand_rb0_pins_a>;
+	status = "okay";
+
+	nand@0 {
+		reg = <0>;
+		allwinner,rb = <0>;
+		nand-ecc-mode = "soft_bch";
+	};
+};