powerpc/ddr: fix fsl_ddr_get_dimm_params compile error

fsl_ddr_get_dimm_params() should be wrapped by
CONFIG_SYS_DDR_RAW_TIMING, otherwise, when using fixed_sdram() instead of
using SPD, it will cause compile error.

Signed-off-by: Shaohui Xie <Shaohui.Xie@freescale.com>
Acked-by: York Sun <yorksun@freescale.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>