mtd: nand: kirkwood: enable BCH ECC and add support for dynamically switching ECC...

mtd: nand: kirkwood: enable BCH ECC and add support for dynamically switching ECC mode

For certain NAND chips it is favourable or required to support 4-bit
ECC schemes. The CPU uses an RS-ECC scheme for which no decoder could
be found and is not worthwile being developed only for this purpose.

Thus enable support for SOFT_BCH ECC which is fully supported by
U-Boot.

Add a command for switching between ECC modes to be able to write a
bootable image and for compatibility with existing Linux
implementations.