gpio/omap: use gpiolib API to mark a GPIO used as an IRQ

The OMAP GPIO driver keeps track about GPIO pins that are
used as IRQ lines for two reasons:

1) To prevent GPIO banks to be disabled while one of their
   GPIO pins are only used as an interrupt line.

2) To not allow another caller to set the GPIO pin as output.

Now gpiolib has an API to mark GPIO pins as used as IRQ lines
so the GPIO core only allows to set as output GPIO pins not
tied to an IRQ. So there is no need to have custom code for 2).

The IRQ usage still has to be maintained locally for 1) though.

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>