kirkwood_nand: allow usage of NAND_ECC_SOFT_BCH

If CONFIG_NAND_ECC_BCH is set use 4-bit error correction code instead of
the 1-bit error correction code on the NAND device.

Signed-off-by: Gerlando Falauto <gerlando.falauto@keymile.com>
Signed-off-by: Holger Brunck <holger.brunck@keymile.com>
cc: Valentin Longchamp <valentin.longchamp@keymile.com>
cc: Prafulla Wadaskar <prafulla@marvell.com>
Acked-by: Prafulla Wadaskar <prafulla@marvell.com>
Acked-by: Scott Wood <scottwood@freescale.com>