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

[Lu jicong]

I'm not satisfied with this result. All of FriendlyARM's official system images use 800Mhz DDR binary.
So they shouldn't found any problem when some boards' DRAM cannot run at 933Mhz.
Although they tested them using 933Mhz DDR binary, my board will still pass the test.
But now TPL initialized DRAM cannot run stably, so I can tell TPL DRAM config must be more strict
than DDR binary.

Before I add openwrt support for NanoPC T4, there is no public linux OS using TPL to initialize
DRAM for NanoPC T4, so there is probably not many people tested TPL. We should make sure
every qualified board can run U-Boot without probelms, shouldn't we?