mtd: update the ABI document about the ecc step size

We add a new sys node for ecc step size. So update the ABI document about it.

Signed-off-by: Huang Shijie <b32955@freescale.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
[Brian: edited description, modified 'ecc_strength']
Signed-off-by: Brian Norris <computersforpeace@gmail.com>

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>