OMAP: dmtimer: add missing include

After commit caf64f2fdc48472995d40656eb1a75524c464447 ("omap: Make a subset
of dmtimer functions into inline functions"),
arch/arm/plat-omap/include/plat/dmtimer.h is missing an include of linux/io.h
- add it.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Tony Lindgren <tony@atomide.com>

arch/arm/plat-omap/include/plat/dmtimer.h

@@ -34,6 +34,7 @@
 
 #include <linux/clk.h>
 #include <linux/delay.h>
+#include <linux/io.h>
 
 #ifndef __ASM_ARCH_DMTIMER_H
 #define __ASM_ARCH_DMTIMER_H