Add support for the hardware ECC BCH engine.

Please mind that this engine has an important limitation:
BCH implementation does not inform the user when an uncorrectable ECC
error occurs. To workaround this, we avoid using the hardware engine
in the read path and do the computation with the software BCH
implementation, which is faster than mixing hardware (for correction)
and software (for verification).
Signed-off-by: NMiquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: NBoris Brezillon <boris.brezillon@collabora.com>
Link: https://lore.kernel.org/linux-mtd/20200519074549.23673-9-miquel.raynal@bootlin.com