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: R Sricharan <r.sricharan@ti.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
[tony@atomide.com: updated comments]
Signed-off-by: Tony Lindgren <tony@atomide.com>