board: sama5d3_xplained: Fix SPL DTB read from NAND

SPL boot cannot find dtb if CONFIG_SPL_SEPARATE_BSS is disabled :

CONFIG_SPL_SEPARATE_BSS=n

RomBOOT
<debug_uart> Missing DTB
 ### ERROR ### Please RESET the board ###
RomBOOT

CONFIG_SPL_SEPARATE_BSS=y

RomBOOT
<debug_uart>
U-Boot SPL 2019.04-linux4sam_6.2-icp-dirty (Feb 24 2020 - 15:34:35 +0100)
Trying to boot from NAND
<debug_uart>

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