mtd: nand: omap2: use gpmc provided irqs

GPMC platform initialization provides it's clients
with interrupts that can be used through struct
resource. Make use of it for irq mode functionality.

Also now write protect disable is done by GPMC,
hence remove it.

Signed-off-by: Afzal Mohammed <afzal@ti.com>
Acked-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
[tony@atomide.com: updated to fix new warnings introduced]
Signed-off-by: Tony Lindgren <tony@atomide.com>