ARM/imx/epit: Migrate to new 'set-state' interface

Migrate imx 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 drop 'clockevent_mode': It was caching the last state of the
clockevent device. The same behavior can be achieved by using
clockevents state helpers. These helpers are only required for oneshot
mode as shutdown/resume wouldn't be done twice by the core.

Cc: Shawn Guo <shawnguo@kernel.org>
Cc: Sascha Hauer <kernel@pengutronix.de>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>