mtd: nand: enable software BCH ECC in nand simulator

This patch adds option 'bch' to nandsim, which can be used to enable
software BCH ECC (introduced in previous patches) and select BCH error
correction capability.

Signed-off-by: Ivan Djelic <ivan.djelic@parrot.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>