mtd: nand: add new enum for storing ECC algorithm

Our nand_ecc_modes_t is already a bit abused by value NAND_ECC_SOFT_BCH.
This enum should store ECC mode only and putting algorithm details there
is a bad idea. It would result in too many values impossible to support
in a sane way.

To solve this problem let's add a new enum. We'll have to modify all
drivers to set it properly but once it's done it'll be possible to drop
NAND_ECC_SOFT_BCH. That will result in a cleaner design and more
possibilities like setting ECC algorithm for hardware ECC mode.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>