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

Migrate metag_generic 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.

We weren't doing anything in the ->set_mode() callback. Even the WARN()
for periodic or unused modes isn't required anymore as the core is
taking care of that now. So, this patch doesn't provide any set-state
callbacks.

Cc: James Hogan <james.hogan@imgtec.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>