arm64: rockchip: rk3399-puma: add DDR3-1333 timings

For the initial validation of the RK3399-Q7 (Puma), the DDR3 has been
clocked at 666MHz (i.e. DDR3-1333) using the same (safe) settings as
used in Rockchip's MiniLoader.
Signed-off-by: NPhilipp Tomsich <philipp.tomsich@theobroma-systems.com>
Acked-by: NSimon Glass <sjg@chromium.org>