dt-bindings: display: sitronix, st7735r: Fix backlight in example

The backlight property was lost during conversion to yaml in commit
abdd9e37 ("dt-bindings: display: sitronix,st7735r: Convert to DT schema").
Put it back.

Fixes: abdd9e37 ("dt-bindings: display: sitronix,st7735r: Convert to DT schema")
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: David Lechner <david@lechnology.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211124150757.17929-2-noralf@tronnes.org

Documentation/devicetree/bindings/display/sitronix,st7735r.yaml

@@ -72,6 +72,7 @@ examples:
                     dc-gpios = <&gpio 43 GPIO_ACTIVE_HIGH>;
                     reset-gpios = <&gpio 80 GPIO_ACTIVE_HIGH>;
                     rotation = <270>;
+                    backlight = <&backlight>;
             };
     };