dt-bindings: Document the Rockchip MIPI RX D-PHY bindings

Add DT bindings documentation for Rockchip MIPI D-PHY RX

Signed-off-by: Jacob Chen <jacob2.chen@rock-chips.com>
Reviewed-by: Rob Herring <robh@kernel.org>
[update for upstream]
Signed-off-by: Helen Koike <helen.koike@collabora.com>

Series-changes: 7
- updated doc with new design and tested example