rk3399: dram: use PMU M0 to do ddr frequency scaling

rk3399: dram: use PMU M0 to do ddr frequency scaling

We used dcf do ddr frequency scaling, but we just include a dcf
binary, it hard to maintain later, we have M0 compile flow in ATF,
and M0 can also work for ddr frequency scaling, so let's use it.

Signed-off-by: Lin Huang <hl@rock-chips.com>
Signed-off-by: Xing Zheng <zhengxing@rock-chips.com>