nios2/time: Migrate to new 'set-state' interface

Migrate nios2 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: Ley Foon Tan <lftan@altera.com>
Cc: Tobias Klauser <tklauser@distanz.ch>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Dmitry Torokhov <dtor@chromium.org>
Cc: nios2-dev@lists.rocketboards.org
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Ley Foon Tan <lftan@altera.com>