Migrate keystone 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 pass the mode-mask to keystone_timer_config() instead of the mode
as mode macro's aren't valid anymore.

Cc: Santosh Shilimkar <ssantosh@kernel.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>