clockevents/drivers/sh_mtu2: Migrate to new 'set-state' interface

Migrate sh_mtu2 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: Magnus Damm <damm+renesas@opensource.se>
Cc: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Cc: Paul Mundt <lethal@linux-sh.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>