gpio/mxc: add .to_irq for gpio chip

It adds .to_irq support for gpio chip, so that __gpio_to_irq in
gpiolib becomes usable.

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