dt-bindings: arm: rockchip: Add NanoPi R6 series boards

Add support for NanoPi R6 series boards that based on RK3588S.

NanoPi R6S basically has:
- USB3
- USB2
- eMMC
- 2x 2.5GBe & 1x 1GBe ethernet
- HDMI
- SD card support

Unlike R6S variant, NanoPi R6C has PCIe M.2 M-key instead of 1x 2.5GBe.
Signed-off-by: Muhammed Efe Cetin <efectn@protonmail.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/30c1c7eac02cd32b76edb77572523f6ad8de89fb.1703934548.git.efectn@protonmail.comSigned-off-by: Heiko Stuebner <heiko@sntech.de>

Documentation/devicetree/bindings/arm/rockchip.yaml

@@ -224,6 +224,13 @@ properties:
               - friendlyarm,nanopi-r5s
           - const: rockchip,rk3568
 
+      - description: FriendlyElec NanoPi R6 series boards
+        items:
+          - enum:
+              - friendlyarm,nanopi-r6c
+              - friendlyarm,nanopi-r6s
+          - const: rockchip,rk3588s
+
       - description: FriendlyElec NanoPC T6
         items:
           - const: friendlyarm,nanopc-t6