Migrate dummy_timer driver to the new 'set-state' interface provided by
clockevents core, the earlier 'set-mode' interface is marked obsolete
now.

We weren't doing anything in the ->set_mode() callback. So, this patch
doesn't provide any set-state callbacks.

Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: NViresh Kumae <viresh.kumar@linaro.org>
Signed-off-by: NDaniel Lezcano <daniel.lezcano@linaro.org>