rockchip: rk3399: Add ROCKCHIP_DEVICE_SETTINGS to set env

Some boards use different stdio environment variables from the default.
Provide a #define for this which can be set before including the header
file.

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