mtd: rawnand: Add a kernel doc to the ECC algorithm enumeration

Before moving it to the generic raw NAND core, ensure the enumeration
is properly described.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://lore.kernel.org/linux-mtd/20200827085208.16276-2-miquel.raynal@bootlin.com