rockchip: dts: rk3399: enable HDMI output in the DTS

This commit enables HDMI output in the DTS by adding the necessary
nodes to vopl/vopb and by adding the HDMI node.

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