mtd: nand: pxa3xx: Use ECC strength and step size devicetree binding

This commit adds support for the user to specify the ECC strength
and step size through the devicetree. We keep the previous behavior,
when there is no DT parameter provided.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>