ARM: dts: at91: sama5d4_xplained: fix addressable nand flash size

sama5d4_xplained comes with a 4Gb NAND flash. Increase 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>