clockevents/drivers/timer-imx-gpt: Migrate to new 'set-state' interface

Migrate timer-imx-gpt 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:
- 'imx_timer.cem': 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.

- 'clock_event_mode_label': CLOCK_EVT_MODE_* shouldn't be used anymore
  by drivers. The prints are modified to print the set-state functions
  name now to debug the driver.

Cc: Shawn Guo <shawn.guo@linaro.org>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>