dt-bindings: Add doc for rk3308-evb

Add compatible for RK3308 Evaluation board

Signed-off-by: Andy Yan <andy.yan@rock-chips.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20191021084642.28562-1-andy.yan@rock-chips.com


Signed-off-by: Heiko Stuebner <heiko@sntech.de>