Fix the following sparse warning:

arch/arm/mach-imx/irq-common.c:24:5: warning: symbol 'mxc_set_irq_fiq' was not declared. Should it be static?
Signed-off-by: NFabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: NShawn Guo <shawn.guo@linaro.org>