mpc83xx: Improve the performance of DDR memory

mpc83xx: Improve the performance of DDR memory

modify the CAS timings. my understanding is that these
settings decrease various wait times in the DDR interface.
Because these wait times are in clock cycles, and the DDR
clock on the 8315 RDB runs slower than on some other 83xx
platforms, we can dial down these values without a problem,
thereby decreasing the latency of memory a little.

Signed-off-by: Howard Gregory <Greg.Howard@freescale.com>
Signed-off-by: Dave Liu <daveliu@freescale.com>
Signed-off-by: Kim Phillips <kim.phillips@freescale.com>