clk: mmp: add mmp private gate clock

Some SOCes have this kind of the gate clock
1. There are some bits to control the gate not only one bit.
2. It is not always that "1" is to enable while "0" is to disable
   when write register.

So we have to define the "mask", "enable_val", "disable_val" for
this kind of gate clock.

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>