dt-bindings: Add doc for Firefly ROC-RK3308-CC board

Add compatible for Firefly ROC-RK3308-CC board.

Signed-off-by: Andy Yan <andy.yan@rock-chips.com>
Link: https://lore.kernel.org/r/20191030072648.29738-1-andy.yan@rock-chips.com


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