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