sam9x60.h: Fix Galois Field Table offsets

Because ATMEL_BASE_ROM is defined to 0x100000, it already points
to the begin of the index table for 512 byte sectors correction.
Thus its offset must be zero and the index of the table for 1024
byte sectors must start at offset 0x8000.

Signed-off-by: Kai Stuhlemmer (ebee Engineering) <kai.stuhlemmer@ebee.de>
[ta: update commit message]
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>