powerpc: call of_clk_init() from time_init()

So the boards which has COMMON_CLK enabled don't have to
invoke this in its board specific file.

Signed-off-by: Kevin Hao <haokexin@gmail.com>
Acked-by: Scott Wood <scottwood@freescale.com>
Acked-by: Michael Turquette <mturquette@linaro.org>
Signed-off-by: Michael Turquette <mturquette@linaro.org>