gpio: omap: convert driver to use gpiolib irqchip

Converts the GPIO OMAP driver to register its chained irq
handler and irqchip using the helpers in the gpiolib core.

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Tested-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>