dt-bindings: arm: rockchip: Update ROCKPi 4 binding

ROCKPi 4 has 3 variants of hardware platforms called
ROCKPi 4A, 4B, and 4C.

- ROCKPi 4A has no Wif/BT.
- ROCKPi 4B has AP6256 Wifi/BT, PoE.
- ROCKPi 4C has AP6256 Wifi/BT, PoE, miniDP, USB Host enabled
  GPIO pin change compared to 4B, 4C

So, update the existing ROCKPi 4 binding to support
ROCKPi 4A/B/C hardware platforms.
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200807094826.12019-1-jagan@amarulasolutions.comSigned-off-by: Heiko Stuebner <heiko@sntech.de>

Documentation/devicetree/bindings/arm/rockchip.yaml

@@ -430,8 +430,12 @@ properties:
           - const: radxa,rock
           - const: rockchip,rk3188
 
-      - description: Radxa ROCK Pi 4
+      - description: Radxa ROCK Pi 4A/B/C
         items:
+          - enum:
+              - radxa,rockpi4a
+              - radxa,rockpi4b
+              - radxa,rockpi4c
           - const: radxa,rockpi4
           - const: rockchip,rk3399