ARM/ixp4xx/timer: Migrate to new 'set-state' interface

Migrate ixp4xx 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.

NOTE: ixp4xx_set_{oneshot|periodic} don't perform read operation on
'IXP4XX_OSRT1' register anymore.

Cc: Imre Kaloz <kaloz@openwrt.org>
Acked-by: Krzysztof Hałasa <khalasa@piap.pl>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>