dt-bindings: display: ti,tdp158: Add missing reg constraint

Device can be used over I2C bus, so it documents 'reg' property, however
it misses to constrain it to actual I2C address.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20250820-dt-bindings-display-v2-2-91e2ccba3d4e@linaro.org


Signed-off-by: Rob Herring (Arm) <robh@kernel.org>