rockchip: dts: firefly using ddr3 1600

According to my test, some of firefly-rk3399 hang after dram init
when using ddr3-1333 config, while using ddr3-1600 config works
for all the board I have test.

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>