rockchip: add u-boot specific dts for rk3368 based boards

Device drivers like debug serial, dmc should be enabled before
relocation, so add u-boot.dtsi files to contain devices that
should be marked as dm-pre-reloc.

Signed-off-by: Andy Yan <andy.yan@rock-chips.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>