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

Migrate mxs 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:
- 'mxs_clockevent_mode': as we have helpers available from core for the
  same.
  same state twice and so perhaps the check wasn't required.
- 'clock_event_mode_label': CLOCK_EVT_MODE_* shouldn't be used anymore
  by drivers and it was used just to print old-state:new-state. The
  debug prints are called from mxs_irq_clear() now based on the
  state-name passed to it. The printed name will be same for shutdown
  and resume states as they use the same callback pointer.

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