mtd: nand: sunxi: Set serial access mode correctly

Replaces the hard coded "always use EDO" policy with that prescribed
by the ONFI 3.1 specification that EDO mode should always be used if tRC
is below 30ns.

Signed-off-by: Roy Spliet <r.spliet@ultimaker.com>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>