Fix the incorrect DDR clk freq reporting on 8536DS

On 8536DS board, When the DDR clk is set async mode(SW3[6:8] != 111),
The display is still sync mode DDR freq. This patch try to fix
this. The display DDR freq is now the actual freq in both
sync and async mode.

Signed-off-by: Jason Jin <Jason.jin@freescale.com>