kirkwood: implement kw_sdram_size_adjust

kirkwood: implement kw_sdram_size_adjust

Size of the SDRAM chips might differ between any two (otherwise
identical) instances of the same board.

So add a function kw_sdram_size_adjust() which reads out the current
ram size for a given bank, and adjusts the Kirkwood's SDRAM window size
register accordingly.

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>