unicore/time: Migrate to new 'set-state' interface

Migrate unicore 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.

We weren't doing anything for PERIODIC or RESUME modes and so their
callbacks aren't implemented.

Cc: Guan Xuetao <gxt@mprc.pku.edu.cn>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>