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>

arch/arm/mach-imx/avic.c

@@ -51,7 +51,7 @@
 
 #define AVIC_NUM_IRQS 64
 
-void __iomem *avic_base;
+static void __iomem *avic_base;
 static struct irq_domain *domain;
 
 static u32 avic_saved_mask_reg[2];