mtd: nand: xway: add missing write_buf and read_buf to nand driver

This driver needs a special write_buf and read_buf function, because we
have to read from a specific address to tell the controller this is a
read from the nand controller.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>