mtd: nand: Let ECC engines advertize the exact number of steps

This is an information that might be useful for specific uses, so
export it, which might avoid having to guess the number of steps when
necessary.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Tested-by: Adam Ford <aford173@gmail.com> #logicpd Torpedo
Link: https://lore.kernel.org/linux-mtd/20210127203020.9574-2-miquel.raynal@bootlin.com