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>