driver/ddr/fsl: Adjust timing_cfg_0 to better support two DDR slots

Increase write-to-write and read-to-read turnaround time for two-slot DDR
configurations. Previously only quad-rank and two dual-rank configurations
have this additional turnaround time. A recent test on two single-rank
DIMMs shows the shorter additional turnaround time is also needed.

Signed-off-by: York Sun <yorksun@freescale.com>