Calculate NAND timing values more accurately on S3C2410, 2412, and
2440, whose registers differ slightly:

Enforce the TACLS max value as 3 on 2440 but 7 on 2410/2412.

Also, make it possible to specify a value of 0 on 2412/2440.

Signed-off-by: Matt Reimer <mreimer@vpop.net>