dt-bindings: add doc for Firefly ROC-RK3399-PC-PLUS

Add devicetree binding documentation for the Firefly ROC-RK3399-PC-PLUS.

Signed-off-by: Levin Du <djw@t-chip.com.cn>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210628035402.16812-2-djw@t-chip.com.cn


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