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

Migrate SPEAr 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.

Also kill the unnecessary forward declaration of set-mode routine by
moving 'struct clock_event_device clkevt' towards the bottom of the file.

Cc: Shiraz Hashim <shiraz.linux.kernel@gmail.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>