dt-bindings: arm: sunxi: add binding for PineTab tablet

Add the device tree binding for Pine64's PineTab tablet, which uses
Allwinner A64 SoC.
Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>

Documentation/devicetree/bindings/arm/sunxi.yaml

@@ -636,6 +636,11 @@ properties:
           - const: pine64,pinebook
           - const: allwinner,sun50i-a64
 
+      - description: Pine64 PineTab
+        items:
+          - const: pine64,pinetab
+          - const: allwinner,sun50i-a64
+
       - description: Pine64 SoPine Baseboard
         items:
           - const: pine64,sopine-baseboard