mtd: Move onenand code base to drivers/mtd/nand/onenand

Move onenand code base to the drivers/mtd/nand directory in the hope
that someday someone will patch it to use the generic NAND helpers.
If it never happens, at least we'll have all NAND related support in a
single directory and not spread over the drivers/mtd/ directory.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>