mtd: nand: make mtk_ecc.c a separated module

this code will be used in mediatek snfi spi-mem controller with
pipelined ECC engine.

Signed-off-by: Chuanhong Guo <gch981213@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20220424032527.673605-2-gch981213@gmail.com