ARM: mach-imx: avic: Staticize *avic_base

Fix the following sparse warning:

arch/arm/mach-imx/avic.c:54:14: warning: symbol 'avic_base' 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>