mtd: rawnand: add Reed-Solomon error correction algorithm

Add Reed-Solomon (RS) to the enumeration of ECC algorithms.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Reviewed-by: Boris Brezillon <boris.brezillon@bootlin.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>