of_mtd: Add helpers to get ECC strength and ECC step size

This commit adds simple helpers to obtain the devicetree properties
that specify the ECC strength and ECC step size to use on a given
NAND controller.

Acked-by: Boris BREZILLON <b.brezillon.dev@gmail.com>
Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>