doc: mtd: nand: update examples to use mtd_to_nand()

mtd_to_nand() has been introduced to hide accesses to mtd->priv.
All NAND controller drivers should use it instead of directly accessing
the ->priv field.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>