ppc4xx: Enable overwriting of default scan window for IBM DDR2 controller

This patch makes it possible to overwrite the default auto-calibration
scan window (SDRAM_WRDTR.[WDTR], SDRAM_CLKTR.[CKTR] values) with
board specific values. The parameters of the weak default function are
corrected as well. This way we don't need the casts any more.

This feature will be used by an upcoming PPC460GT board port.

Signed-off-by: Stefan Roese <sr@denx.de>