Convert CONFIG_NAND_OMAP_ECCSCHEME to Kconfig

The values of CONFIG_NAND_OMAP_ECCSCHEME map to the enum in
include/linux/mtd/omap_gpmc.h for valid ECC schemes.  Make which one we
will use be a choice statement, enumerating the ones which we have
implemented.

Signed-off-by: Tom Rini <trini@konsulko.com>