Blackfin arch: Fix bug - 561 SMP kernel can't boot from jffs2

bss_l2 section is garbage when the data in this section is used by
_bfin_relocate_l1_mem, so move the zero out function ahead.

Signed-off-by: Graf Yang <graf.yang@analog.com>
Signed-off-by: Bryan Wu <cooloney@kernel.org>