dt-bindings: display: mitsubishi,aa104xd12: Adjust allowed and required properties

dt-bindings: display: mitsubishi,aa104xd12: Adjust allowed and required properties

The Mitsubishi aa104xd12 panel requires an external backlight driver
circuit, so allow the "backlight" property.

There are users of this panel without a vcc-supply, so it shouldn't be
required.

Signed-off-by: "Rob Herring (Arm)" <robh@kernel.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20250225210316.3043357-2-robh@kernel.org
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>