dt-bindings: add document for rk3036-vop

Cc: Rob Herring <robh+dt@kernel.org>
Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Ian Campbell <ijc+devicetree@hellion.org.uk>
Cc: Kumar Gala <galak@codeaurora.org>
Cc: devicetree@vger.kernel.org

Signed-off-by: Mark Yao <mark.yao@rock-chips.com>
Acked-by: Rob Herring <robh@kernel.org>