Blackfin arch: fix bug - Dmacopy failed in BF537-STAMP

Dmacopy failed in BF537-STAMP when copy from SRAM to SDRAM and kernel
will reboot automatically.

Fixing by doing a SSYNC before mucking with DMA registers

Signed-off-by: Mike Frysinger <vapier.adi@gmail.com>
Signed-off-by: Bryan Wu <cooloney@kernel.org>