dt-bindings: mtd: Document nand-ecc-placement

This optional property defines where the ECC bytes are expected to be
stored. No value defaults to an unknown location, while these
locations can be explicitly set to OOB or interleaved depending if
the ECC bytes are entirely stored in the OOB area or mixed with
regular data in the main area (also sometimes referred as
"syndrome").

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/linux-mtd/20200827085208.16276-6-miquel.raynal@bootlin.com