mtd: docg3: add device-tree documentation

Add documentation for the sandisk docg3 chip.
Signed-off-by: NRobert Jarzmik <robert.jarzmik@free.fr>
Cc: devicetree@vger.kernel.org
Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: NBrian Norris <computersforpeace@gmail.com>

Documentation/devicetree/bindings/mtd/diskonchip.txt

+M-Systems and Sandisk DiskOnChip devices
+
+M-System DiskOnChip G3
+======================
+The Sandisk (formerly M-Systems) docg3 is a nand device of 64M to 256MB.
+
+Required properties:
+ - compatible: should be "m-systems,diskonchip-g3"
+ - reg: register base and size
+
+Example:
+	docg3: flash@0 {
+		compatible = "m-systems,diskonchip-g3";
+		reg = <0x0 0x2000>;
+	};