ARM: OMAP2+: gpmc: update nand register helper

Provide helper function for updating NAND register details for
the necessary chip select. NAND drivers platform data can be
updated with this information so that NAND driver can handle
GPMC NAND operations by itself.

Signed-off-by: Afzal Mohammed <afzal@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>