Define the galois index table offset in chip head file. So user do not need
to set by himself. Driver will set it correctly according to sector_size.

Signed-off-by: Josh Wu <josh.wu@atmel.com>
Acked-by: Scott Wood <scottwood@freescale.com>
[rebased on master]
Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>