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

Migrate openrisc 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 in the ->set_mode() callback. So, this patch
doesn't provide any set-state callbacks.

Cc: Jonas Bonn <jonas@southpole.se>
Cc: linux@lists.openrisc.net
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>