ARM/omap2/timer: Migrate to new 'set-state' interface

Migrate omap2 driver to the new 'set-state' interface provided by
clockevents core, the earlier 'set-mode' interface is marked obsolete
now.

This also enables us to implement callbacks for new states of clockevent
devices, for example: ONESHOT_STOPPED.

Acked-by: Santosh Shilimkar <ssantosh@kernel.org>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>