dt-bindings: display: sun4i: Fix D1 pipeline count

When adding the bindings for the D1 display engine, I missed the
condition for the number of pipelines. D1 has two mixers, so it
will have two pipeline references.

Fixes: ae5a5d26 ("dt-bindings: display: Add D1 display engine compatibles")
Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Signed-off-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Link: https://lore.kernel.org/r/20220702032921.22433-1-samuel@sholland.org

Documentation/devicetree/bindings/display/allwinner,sun4i-a10-display-engine.yaml

@@ -94,6 +94,7 @@ if:
           - allwinner,sun8i-a83t-display-engine
           - allwinner,sun8i-r40-display-engine
           - allwinner,sun9i-a80-display-engine
+          - allwinner,sun20i-d1-display-engine
           - allwinner,sun50i-a64-display-engine
 
 then: