mtd: nand: move nand_check_erased_ecc_chunk() to nand/core

The check function for bitflips in erased blocks will be needed
by the Realtek ECC engine driver (which is currently under
development). Right now it is located in raw/nand_base.c.
While this is sufficient for the current usecases, there is
no real dependency for an ECC engine on the raw nand library.

Move the function over to a more generic place in core library.

Suggested-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>