mtd: nand: mxs_nand: add support for specific ECC strength

Add support for specified ECC strength/size using device tree
properties nand-ecc-strength/nand-ecc-step-size.

This aligns behavior with the mainline driver, such that:
- If fsl,use-minimal-ecc is requested it will use data from
  data sheet/ONFI. If this is not available the driver will fail.
- If nand-ecc-strength/nand-ecc-step-size are specified those
  value will be used.
- By default maximum possible ECC strength is used

Signed-off-by: Stefan Agner <stefan.agner@toradex.com>