Migrate w90x900 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.

Cc: Wan ZongShun <mcuos.com@gmail.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>