Fix the following sparse warning:

arch/arm/mach-imx/iomux-imx31.c:43:15: warning: symbol 'mxc_pin_alloc_map' 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>