omap: gpmc: 'nandecc sw' can use HAM1 or BCH8

omap: gpmc: 'nandecc sw' can use HAM1 or BCH8

The 'nandecc sw' command selects a software-based error correction
algorithm.  By default, this is OMAP_ECC_HAM1_CODE_SW but some
platforms use OMAP_ECC_BCH8_CODE_HW_DETECTION_SW as their
software-based correction algorithm.  Allow a user to be specific e.g.
 # nandecc sw <hamming|bch8>
where 'hamming' is still the default.

Note: we don't just use CONFIG_NAND_OMAP_ECCSCHEME as it might be set
      to a hardware-based ECC scheme---a little strange when the user
      has requested 'sw' ECC.

Signed-off-by: Ash Charles <ashcharles@gmail.com>