ARM: OMAP: Move dmtimer.h out of plat-omap

The header file is currently under plat-omap directory
under arch/omap. Move this out to an accessible place.

No Code changes done to the header file and renamed to timer-ti-dm.h.

Signed-off-by: Keerthy <j-keerthy@ti.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Tested-by: Ladislav Michl <ladis@linux-mips.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>