mtd: nand: simplify nand_dt_init() usage

nand_dt_init() function requires 3 arguments where it actually needs one
(dn and mtd can both be retrieved from chip). Drop these parameters.

Testing for dn != NULL inside nand_dt_init() also helps simplifying the
caller code.

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