ARM: OMAP2+: gpmc-nand: Modify Interrupt handling

Now GPMC provides its client with interrupts that can be handled
using the standard interrupt API. Modify GPMC NAND setup to work
with it.

Also disable write protect in GPMC code, so that NAND driver can
be ignorant of GPMC configuration.

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