clk: mmp: add spin lock for clk-frac

The register used by clk-frac may be shared with
other clocks.
So it needs to use spin lock to protect the register
access.

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>