gpio/mxc: add chained_irq_enter/exit() to mx2_gpio_irq_handler

Similar to commit

	0e44b6ec (gpio/mxc: add chained_irq_enter/exit() to mx3_gpio_irq_handler())

. It doesn't seem to be critical to make the irqs work, but still it is
more correct.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>