dt-bindings: arm: sunxi: document Transpeed 8K618-T board name

The Transpeed 8K618-T TV box is a Chinese Android TV box, using the
Allwinner H618 SoC.

Add the board/SoC compatible string pair to the list of known boards.
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20231214015312.17363-3-andre.przywara@arm.comSigned-off-by: Jernej Skrabec <jernej.skrabec@gmail.com>

Documentation/devicetree/bindings/arm/sunxi.yaml

@@ -868,6 +868,11 @@ properties:
           - const: topwise,a721
           - const: allwinner,sun4i-a10
 
+      - description: Transpeed 8K618-T
+        items:
+          - const: transpeed,8k618-t
+          - const: allwinner,sun50i-h618
+
       - description: Utoo P66
         items:
           - const: utoo,p66