nios2: BSS should be cleared only after board_init_f

As dtb in u-boot-dtb.bin overlapped the BSS section,
we should delay the clearing of BSS until dtb is relocated
in board_init_f().

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>
Acked-by: Marek Vasut <marex@denx.de>