ARM: OMAP2+: Remove empty io.h

The file arch/arm/mach-omap2/io.h is empty, so we can remove it.
Signed-off-by: NThomas Weber <weber@corscience.de>
Acked-by: NKevin Hilman <khilman@ti.com>
Signed-off-by: NTony Lindgren <tony@atomide.com>