dt-bindings: arm: rockchip: Document Radxa ROCK 4C+

Document the dt-bindings for Radxa ROCK 4C+ SBC.

Key differences of 4C+ compared to previous ROCK Pi 4.
- Rockchip RK3399-T SoC
- DP from 4C replaced with micro HDMI 2K@60fps
- 4-lane MIPI DSI with 1920*1080
- RK817 Audio codec

Also, an official naming convention from Radxa mention to remove
Pi from board name, so this 4C+ is named as Radxa ROCK 4C+ not
Radxa ROCK Pi 4C+.
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Link: https://lore.kernel.org/r/20220902065057.97425-1-jagan@amarulasolutions.comSigned-off-by: Heiko Stuebner <heiko@sntech.de>

Documentation/devicetree/bindings/arm/rockchip.yaml

@@ -547,6 +547,11 @@ properties:
           - const: radxa,rockpi4
           - const: rockchip,rk3399
 
+      - description: Radxa ROCK 4C+
+        items:
+          - const: radxa,rock-4c-plus
+          - const: rockchip,rk3399
+
       - description: Radxa ROCK Pi E
         items:
           - const: radxa,rockpi-e