common: Add a prototype for s_init()

When compiling with W=1 the following warning is observed:

arch/arm/mach-imx/mx6/soc.c:590:6: warning: no previous prototype for ‘s_init’ [-Wmissing-prototypes] void s_init(void)

Remove this warning by adding the function prototype into include/common.h file.

Signed-off-by: Diego Dorta <diego.dorta@nxp.com>

include/common.h

@@ -514,6 +514,8 @@ int	is_core_valid (unsigned int);
  */
 int arch_cpu_init(void);
 
+void s_init(void);
+
 int	checkcpu      (void);
 int	checkicache   (void);
 int	checkdcache   (void);