arm/kirkwood/nand: allow forced disabling for subpage writes

Make it configurable to disable subpage writes like the DaVinci NAND
driver already does.

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