board: sama5d3_xplained: Fix uboot size when loaded from NAND by SPL

Uboot size is incorrect.
Uboot SPL use CONFIG_SYS_MONITOR_LEN to read uboot from NAND : 0x80000
With sama5d3_xplained_nandflash_defconfig : u-boot.bin size is ~800Ko 0xC0000

So I increased size to 1MB : 0x100000

Signed-off-by: Fabien Lehoussel <fabien.lehoussel@medianesysteme.com>