configs: at91: sama5: updated mtdparts variable in bootargs

We have a new demo layout of our sama5 boards for the NAND Flash
memory.
According to this new layout, adjust the mtdparts variable in bootargs
to align with this, which is available at :
http://www.at91.com/linux4sam/bin/view/Linux4SAM/Sama5d3XplainedMainPage#NAND_Flash_demo_Memory_map

,

Based on original work by Wenyou Yang

Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>