clocksource: of: Respect device tree node status

Clocksource devices provided by DT can be disabled (status != "okay").
Instead of registering clocksource drivers for disabled nodes, respect
the device's status by skiping disabled nodes.

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>