omap2+: Reserve clocksource and timesource and initialize dmtimer later

There's no need to initialize the dmtimer framework early.
Just mark the clocksource and timesource as reserved, and
initialize dmtimer with an arch_initcall.

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