arm/km: disable subpage write for km_kirkwood_pci and kmsuv31 target

Newer FLASH types used on these boards don't allow writing of subpages.
So disable subpage write in the NAND driver. Additionally we need to
tell the UBI layer in the kernel that he also should only write 2048
bytes. This is done with an additional command line parameter for the
kernel commandline.

Signed-off-by: Holger Brunck <holger.brunck@keymile.com>
cc: Valentin Longchamp <valentin.longchamp@keymile.com>
cc: Prafulla Wadaskar <prafulla@marvell.com>