configs: at91: Adjust CONFIG_ENV_OFFSET to match sama5 address

In order to have a single ENV_OFFSET to manage, use the same as the sama5 one.
This address matches our NAND flash map available at:
http://www.at91.com/linux4sam/bin/view/Linux4SAM/Sama5d3XplainedMainPage#NAND_Flash_demo_Memory_map



Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
[eugen.hristev@microchip.com: rework on latest version of u-boot]
Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>