clk: mmp: add basic support functions for DT support

In order to support DT for mmp SOC clocks, it defines
some basic APIs which are shared by all mmp SOC clock
units.

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>