ARM/gemini/time: Migrate to new 'set-state' interface

Migrate gemini 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: Hans Ulli Kroll <ulli.kroll@googlemail.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>