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 <gerlando.falauto@keymile.com>
Signed-off-by: Holger Brunck <holger.brunck@keymile.com>
cc: Prafulla Wadaskar <prafulla@marvell.com>
cc: Valentin Longchamp <valentin.longchamp@keymile.com>
cc: Marek Vasut <marex@denx.de>
Acked-by: Prafulla Wadaskar <Prafulla@marvell.com>