omap2+: Use dmtimer macros for clocksource

Use dmtimer macros for clocksource. As with the clockevent,
this allows us to initialize the rest of dmtimer code later on.

Note that eventually we will be initializing the timesource
from init_early so sched_clock will work properly for
CONFIG_PRINTK_TIME.

Signed-off-by: Tony Lindgren <tony@atomide.com>
Reviewed-by: Kevin Hilman <khilman@ti.com>