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

The mx3_gpio_irq_handler() is also called on imx6q which has GIC as
the primary interrupt controller.  As GIC implements the fasteoi flow
control, we need to add chained_irq_enter/exit() to
mx3_gpio_irq_handler() for signaling EOI, otherwise system will hang
whenever there is a gpio irq triggered.

v2: use chained_irq_{enter,exit}()

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>