mtd: nand: move of_get_nand_xxx() helpers into nand_base.c

Now that all drivers go through nand_set_flash_node() to parse the generic
NAND properties, we can move all of_get_nand_xxx() helpers in to
nand_base.c, make them static and remove of_mtd.c and of_mtd.h.

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