mtd: nand: sunxi: Replace failsafe timing cfg with calculated value

The TIMING_CFG register was previously statically set to a magic value
(extracted from Allwinner's BSP) when initializing the NAND controller.
Now that we have more details about the TIMING_CFG register layout
(extracted from the A83 user manual) we can dynamically calculate the
appropriate value for each NAND chip and set it when selecting the
chip.

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>