rockchip: dts: rk3399-puma: add DTS for the DDR3-1866 timing

This adds the DDR3-1866 timing via its own DTS and wires it up.  This
(currently) is not the default timing for the RK3399-Q7 and should be
selected explicitly via the config (CONFIG_DEFAULT_DEVICE_TREE).

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