mtd: nand: xway: add nandaddr to own struct

Instead of using IO_ADDR_W and IO_ADDR_R use an own pointer to the NAND
controller memory area.

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