dt-bindings: arm: sunxi: document Orange Pi Zero 3 board name

The Orange Pi Zero 3 board is an updated version of the Zero 2 board.
It uses a SoC called H618, which just seems to be an H616 with more L2
cache.

Add the board/SoC compatible string pair to the list of known boards.
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Link: https://lore.kernel.org/r/20230804170856.1237202-3-andre.przywara@arm.comSigned-off-by: Jernej Skrabec <jernej.skrabec@gmail.com>

Documentation/devicetree/bindings/arm/sunxi.yaml

@@ -997,4 +997,9 @@ properties:
           - const: xunlong,orangepi-zero2
           - const: allwinner,sun50i-h616
 
+      - description: Xunlong OrangePi Zero 3
+        items:
+          - const: xunlong,orangepi-zero3
+          - const: allwinner,sun50i-h618
+
 additionalProperties: true