Fix the following sparse warnings:

arch/arm/mach-imx/cpu.c:10:6: warning: symbol 'mxc_set_cpu_type' was not declared. Should it be static?
arch/arm/mach-imx/cpu.c:15:6: warning: symbol 'imx_print_silicon_rev' was not declared. Should it be static?
arch/arm/mach-imx/cpu.c:24:13: warning: symbol 'imx_set_aips' 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>