clk: mmp: add pxa168 DT support for clock driver

It adds the DT support for pxa168 clock subsystem.

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>