mtd: onenand: omap2: Decouple DMA enabling from INT pin availability

INT pin (gpio_irq) is not really needed for DMA but only for notification
when a command that needs wait has completed. DMA memcpy can be still used
even without gpio_irq available, so enable it unconditionally.

Signed-off-by: Ladislav Michl <ladis@linux-mips.org>
Reviewed-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Tested-by: Tony Lindgren <tony@atomide.com>
Tested-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Acked-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>