ARM: OMAP: Move dmtimer driver out of plat-omap to drivers under clocksource

Move the dmtimer driver out of plat-omap to clocksource.
So that non-omap devices also could use this.

No Code changes done to the driver file only renamed to timer-ti-dm.c.
Also removed the config dependencies for OMAP_DM_TIMER.

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>
[tony@atomide.com: add select omap_dm_timer for omap16xx]
Signed-off-by: Tony Lindgren <tony@atomide.com>