arm/mxc: use gpiolib helper for gpio_to_irq

As all the users of gpio_to_irq() in static initializers have been
migrated to IMX_GPIO_TO_IRQ, we can start using the standard gpiolib
helper for gpio_to_irq().

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>