omap: Make a subset of dmtimer functions into inline functions

This will allow us to share the code between system timer and
dmtimer device driver code without having to initialize all the
dmtimers early. This change will also make the timer_set_next_event
more efficient as the inline functions will optimize the code
better for the timer reprogramming.

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