Fix the following sparse warning:

arch/arm/mach-mxs/pm.c:37:13: warning: symbol 'mxs_pm_init' 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>