ARM: OMAP2+: Fix omap2+ build error

With CONFIG_OMAP4_ERRATA_I688 enabled, omap2+ build
was broken as below:

arch/arm/kernel/io.c: In function '_memcpy_toio':
arch/arm/kernel/io.c:29: error: implicit declaration of function 'outer_sync'
make[1]: *** [arch/arm/kernel/io.o] Error 1

This was caused by commit 9f97da78 (Disintegrate asm/system.h for ARM).
Signed-off-by: NR Sricharan <r.sricharan@ti.com>
Acked-by: NSantosh Shilimkar <santosh.shilimkar@ti.com>
[tony@atomide.com: updated comments]
Signed-off-by: NTony Lindgren <tony@atomide.com>

arch/arm/include/asm/barrier.h

@@ -2,6 +2,7 @@
 #define __ASM_BARRIER_H
 
 #ifndef __ASSEMBLY__
+#include <asm/outercache.h>
 
 #define nop() __asm__ __volatile__("mov\tr0,r0\t@ nop\n\t");
 
@@ -39,7 +40,6 @@
 #ifdef CONFIG_ARCH_HAS_BARRIERS
 #include <mach/barriers.h>
 #elif defined(CONFIG_ARM_DMA_MEM_BUFFERABLE) || defined(CONFIG_SMP)
-#include <asm/outercache.h>
 #define mb()		do { dsb(); outer_sync(); } while (0)
 #define rmb()		dsb()
 #define wmb()		mb()