mtd: nand: ecc-bch: Create the software BCH engine

Let's continue introducing the generic ECC engine abstraction in the
NAND subsystem by instantiating a first ECC engine: the software
BCH one.

While at it, make a very tidy ecc_sw_bch_init() function and move all
the sanity checks and user input management in
nand_ecc_sw_bch_init_ctx(). This second helper will be called from the
raw RAND core.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200929230124.31491-10-miquel.raynal@bootlin.com