clockevents/drivers/stm32: Migrate to new 'set-state' interface

Migrate stm32 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.

Cc: Maxime Coquelin <mcoquelin.stm32@gmail.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Tested-by: Maxime Coquelin <mcoquelin.stm32@gmail.com>
Acked-by: Maxime Coquelin <mcoquelin.stm32@gmail.com>