ARC: clockevent: Prepare for DT based probe

 - call clocksource_probe()
 - This in turns needs of_clk_init() to be called earlier

Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Noam Camus <noamc@ezchip.com>
[vgupta: broken off from a bigger patch]
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>