MAP: boards: Fix OMAP_GPIO_IRQ usage with gpio_to_irq()

The following commits change gpio-omap to use dynamic
IRQ allocation:

25db711 gpio/omap: Fix IRQ handling for SPARSE_IRQ
384ebe1 gpio/omap: Add DT support to GPIO driver

With dynamic allocation of IRQ the usage of OMAP_GPIO_IRQ
is no longer valid. We must be using gpio_to_irq() instead.

Signed-off-by: Tarun Kanti DebBarma <tarun.kanti@ti.com>
[tony@atomide.com: updated comments]
Signed-off-by: Tony Lindgren <tony@atomide.com>