fsl-ddr: use the 1T timing as default configuration

fsl-ddr: use the 1T timing as default configuration

For light loaded system, we use the 1T timing to gain better
memory performance, but for some heavily loaded system,
you have to add the 2T timing options to board files.

Signed-off-by: Dave Liu <daveliu@freescale.com>
Acked-by: Andy Fleming <afleming@freescale.com>