imx_irq_set_priority() is not used anywhere, so remove it.

This fixes the following sparse warnings:

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