kirkwood: implement kw_sdram_bs_set()

kirkwood: implement kw_sdram_bs_set()

Some boards might be equipped with different SDRAM configurations.
When that is the case, CPU CS Window Size Register (CS[0]n Size)
should be set to the biggest value through board.cfg file; then its
value can be fixed at runtime according to the detected SDRAM size.

Therefore, implement kw_sdram_bs_set().

Signed-off-by: Gerlando Falauto <[email protected]>
Signed-off-by: Holger Brunck <[email protected]>
cc: Prafulla Wadaskar <[email protected]>
cc: Valentin Longchamp <[email protected]>
cc: Marek Vasut <[email protected]>
Acked-by: Prafulla Wadaskar <[email protected]>