ARM: imx: fix imx_init_l2cache storage class

This fixes the following compilation error:

  arch/arm/mach-imx/system.c:101:123: error: static declaration of ‘imx_init_l2cache’ follows non-static declaration
  In file included from arch/arm/mach-imx/system.c:32:0:
  arch/arm/mach-imx/common.h:165:13: note: previous declaration of ‘imx_init_l2cache’ was here
  arch/arm/mach-imx/system.c:101:123: warning: ‘imx_init_l2cache’ defined but not used [-Wunused-function]

Signed-off-by: Vincent Stehlé <vincent.stehle@freescale.com>
Cc: Sascha Hauer <kernel@pengutronix.de>
Cc: Russell King <linux@arm.linux.org.uk>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>