ARM: at91: atmel_nand: pmecc driver will select the galois table by sector size

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>