fsl-ddr: Fix two bugs in the ddr infrastructure

1. wr_lat
   UM said the total write latency for DDR2 is equal to
   WR_LAT + ADD_LAT, the write latency is CL + ADD_LAT - 1.
   so, the WR_LAT = CL - 1;
2. rd_to_pre
   we missed to add the ADD_LAT for DDR2 case.

Reported-by: Joakim Tjernlund <Joakim.Tjernlund@transmode.se>
Signed-off-by: Dave Liu <daveliu@freescale.com>