mtd: spinand: micron: Generalize the OOB layout structure and function names

In order to add new Micron SPI NAND devices, we generalized the OOB
layout structure and function names.

Signed-off-by: Shivamurthy Shastri <sshivamurthy@micron.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200311175735.2007-2-sshivamurthy@micron.com