clockevents/drivers/arm_global_timer: Migrate to new 'set-state' interface

Migrate arm_global_timer driver to the new 'set-state' interface
provided by the 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.

Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Maxime Coquelin <maxime.coquelin@st.com>
Acked-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Cc: Srinivas Kandagatla <srinivas.kandagatla@gmail.com>
Cc: Maxime Coquelin <maxime.coquelin@st.com>
Cc: Patrice Chotard <patrice.chotard@st.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>