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

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

BugLink: https://bugs.launchpad.net/bugs/1981864
[ Upstream commit 471e201f543559e2cb19b182b680ebf04d80ee31 ]

The backlight property was lost during conversion to yaml in commit
abdd9e3705c8 ("dt-bindings: display: sitronix,st7735r: Convert to DT schema").
Put it back.

Fixes: abdd9e3705c8 ("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
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>