dt-bindings: arm: rockchip: add radxa rock3 model a

Add entry for the Radxa ROCK3 Model A board (with the Rockchip
RK3568 SoC).
Signed-off-by: Michael Riesch <michael.riesch@wolfvision.net>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220310210352.451136-2-michael.riesch@wolfvision.netSigned-off-by: Heiko Stuebner <heiko@sntech.de>

Documentation/devicetree/bindings/arm/rockchip.yaml

@@ -545,6 +545,11 @@ properties:
           - const: radxa,rock2-square
           - const: rockchip,rk3288
 
+      - description: Radxa ROCK3 Model A
+        items:
+          - const: radxa,rock3a
+          - const: rockchip,rk3568
+
       - description: Rikomagic MK808 v1
         items:
           - const: rikomagic,mk808