OMAP: dmtimer: add missing include

After commit caf64f2f ("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>