OMAP gpios implement new to_irq()

Make OMAP use the new __gpio_to_irq() hook, to make it easier to
support IRQs coming in from off-chip gpio controllers like the
TWL4030/TPS65930 chip used on OMAP3 boads like Beagleboard.org and
the Gumstix Overo.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Tony Lindgren <tony@atomide.com>