ARM/cns3xxx/timer: Migrate to new 'set-state' interface

Migrate cns3xxx 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.

NOTE: We don't read TIMER1_2_CONTROL_OFFSET register on shutdown
anymore.

Acked-by: Krzysztof Hałasa <khalasa@piap.pl>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>