mtd: rawnand: Move ONFI code to nand_onfi.c

This moves ONFI related code to nand_onfi.c and ONFI related
struct/macros to include/linux/mtd/onfi.h.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>