clk: mmp: add init callback for clk-frac

For the clk-frac, we need to make sure that the initial
clock rate is one item of the table.
If it is not, we use the first item in the table by default.

Signed-off-by: Chao Xie <chao.xie@marvell.com>
Acked-by: Haojian Zhuang <haojian.zhuang@gmail.com>
Signed-off-by: Michael Turquette <mturquette@linaro.org>