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

Migrate pxa 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.

Both oneshot and shutdown modes had exactly same code and so only a
single callback is sufficient now, which will be called for both the
modes.

Cc: Robert Jarzmik <robert.jarzmik@free.fr>
Cc: Russell King <linux@arm.linux.org.uk>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Robert Jarzmik <robert.jarzmik@free.fr>