dt-bindings: mtd: document new "on-die" nand-ecc-mode

A number of NAND chips support a feature called on-die ECC, where the
NAND chip itself is capable of doing error detection and correction. The
new "on-die" value for nand-ecc-mode indicates that we want this
functionality to be used.
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>

Documentation/devicetree/bindings/mtd/nand.txt

@@ -21,7 +21,7 @@ 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".
+		  "hw_oob_first", "on-die".
 		  Deprecated values:
 		  "soft_bch": use "soft" and nand-ecc-algo instead
 - nand-ecc-algo: string, algorithm of NAND ECC.