mtd: add get/set of_node/flash_node helpers

mtd: add get/set of_node/flash_node helpers

Linux commit 28b8b26b308 ("mtd: add get/set of_node/flash_node helpers")

We are going to begin using the mtd->dev.of_node field for MTD device
nodes, so let's add helpers for it. Also, we'll be making some
conversions on spi_nor (and nand_chip eventually) too, so get that ready
with their own helpers.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Reviewed-by: Boris Brezillon <boris.brezillon@free-electrons.com>
[Philippe Reynes: only add function nand_set_flash_node and
nand_get_flash_node because others were already backported]

Signed-off-by: Philippe Reynes <philippe.reynes@softathome.com>