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

Migrate integrator 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.

For oneshot mode the clkevt device was first getting disabled by
clearing TIMER_CTRL_ENABLE bits in TIMER_CTRL register, followed by
clearing TIMER_CTRL_PERIODIC bit. Both these are done with a single
write operation now.

Cc: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>