[PATCH] rockchip: rk3399: nanopc-t4: use 1600MHz sdram config

[Lu jicong]

> Does the 1866M worked for your board before?
1866M ddr blobs from rkbin works fine on my board, but TPL works bad every time.
I have never encountered an exception of this result.

> I think this config should be fine for other people, it has been
> submit for 4 years.
For another tester, this config works fine. But I think the individual difference of
this board or different board revision leads to the result. Maybe it is better to lower
the frequency to ensure more people can use TPL without problems.