mtd: nand: document the NAND controller/NAND chip DT representation

* MTD generic binding

* NAND chip and NAND controller generic binding

NAND controller/NAND chip representation:

The NAND controller should be represented with its own DT node, and all

NAND chips attached to this controller should be defined as children nodes

of the NAND controller. This representation should be enforced even for

simple controllers supporting only one chip.

Mandatory NAND controller properties:

- #address-cells: depends on your controller. Should at least be 1 to

		  encode the CS line id.

- #size-cells: depends on your controller. Put zero unless you need a

	       mapping between CS lines and dedicated memory regions

Optional NAND controller properties

- ranges: only needed if you need to define a mapping between CS lines and

	  memory regions

Optional NAND chip properties:

- nand-ecc-mode : String, operation mode of the NAND ecc mode.

  Supported values are: "none", "soft", "hw", "hw_syndrome", "hw_oob_first",

The interpretation of these parameters is implementation-defined, so not all

implementations must support all possible combinations. However, implementations

are encouraged to further specify the value(s) they support.

Example:

	nand-controller {

		#address-cells = <1>;

		#size-cells = <0>;

		/* controller specific properties */

		nand@0 {

			reg = <0>;

			nand-ecc-mode = "soft_bch";

			/* controller specific properties */

		};

	};