rockchip: dts: rk3399-puma: Add DDR3-1866 timings

With the validation done for DDR3-1866 (i.e. 933 MHz bus clock), we
can now add the timings (rk3399-sdram-ddr3-1866.dtsi) for boards built
with the DDR3-1866 option.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>