ARM: dts: at91: at91sam9x5cm: fix addressable nand flash size

at91sam9x5cm comes with a 2Gb NAND flash. Fix the rootfs size to
match this limit.

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Signed-off-by: Ludovic Desroches <ludovic.desroches@microchip.com>