arm/imx: fix return type of callback passed to of_irq_init()

The of_irq_init() expects the callback passed by .data of of_device_id
return 'int' instead of 'void'.  This patch fixes it to have
irq_init_cb() return the correct value, and in turn have the secondary
interrupt controller (gpio in this case) initialized properly and also
eliminate the error message 'of_irq_init: children remain, but no
parents' which was overlooked before.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>