Fix the following sparse warning:

arch/arm/mach-imx/mm-imx3.c:85:13: warning: symbol 'imx3_init_l2x0' 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>